Okay, so I did small research and I think all problems are solved. A bit of information, maybe someone else will also struggle with similar problem.

1. Using buck/boost converters near signal wires (like probes) might induce noise and cause problems
2. If you struggle with unstable printer work/your voltage readings (in DWC) vary under load (even if PSU ratings are higher than requirements of your setup) check your PSU in diffrent position (horizontal/vertical), if it makes diffrence you probably have problem with cold solder joints - if you are familiar with soldering and electronics - redo solder joints in your PSU - if not, better not to do it yourself.

I did small research for my printer, decided to do some tests with opened enclosure of PSU to measure some things and noticed, when I test it laying flat, there is no problem (in printer PSU is mounted vertically), as soon as I mount it back, problem occured again.
Decided to redo all solder joints on PCB (I couldn't find anything that looked really suspicious) with special attention for filtering sections, added also some lead-solder. After that I don't have any problems like before.
I think problem is solved.

@phaedrux If it's necessary I can do video showing the problem. I might also try updating to 3.4 beta and see results.

@phaedrux said in BL Touch randomly deploys itself after update.:

well one way to check if it's firmware would be to roll back to your previous version.

What kind of bed heater is it?

Can you post your full config?

So I tried downgrading to 3.1.1, checked also 3.2 and 3.2.2 - everything works fine as it should. After updating to 3.3 problem occurs immediately.

@phaedrux It's classic aluminium pcb bed heater similar to one from picture below.

My config is

; Configuration file for Duet WiFi (firmware version 3)
; executed by the firmware on start-up
;
; generated by RepRapFirmware Configuration Tool v3.1.4 on Sun Oct 04 2020 13:36:35 GMT+0200 (czas środkowoeuropejski letni)

; General preferences
G90                                            ; send absolute coordinates...
M83                                            ; ...but relative extruder moves
M550 P"notreallyaprintername"                        ; set printer name
M669 K1                                        ; select CoreXY mode

; Network
M552 S1                                        ; enable network
M586 P0 S1                                     ; enable HTTP
M586 P1 S0                                     ; disable FTP
M586 P2 S0                                     ; disable Telnet

; Drives
M569 P0 S1                                     ; physical drive 0 goes forwards
M569 P1 S1                                     ; physical drive 1 goes forwards
M569 P2 S1                                     ; physical drive 2 goes forwards
M569 P3 S1                                     ; physical drive 3 goes forwards
M584 X0 Y1 Z2 E3                               ; set drive mapping
M350 X16 Y16 Z16 E16 I1                        ; configure microstepping with interpolation
M92 X100.00 Y100.00 Z400.00 E415.00            ; set steps per mm
M566 X600.00 Y600.00 Z12.00 E270.00            ; set maximum instantaneous speed changes (mm/min)
M203 X15000.00 Y15000.00 Z720.00 E7200.00      ; set maximum speeds (mm/min)
M201 X2500.00 Y2500.00 Z500.00 E5000.00         ; set accelerations (mm/s^2)
M906 X700 Y700 Z550 E700 I30                   ; set motor currents (mA) and motor idle factor in per cent
M84 S30                                        ; Set idle timeout

; Axis Limits
M208 X0 Y0 Z0 S1                               ; set axis minima
M208 X200 Y200 Z200 S0                         ; set axis maxima

; Endstops
M574 X1 S1 P"xstop"                            ; configure active-high endstop for low end on X via pin xstop
M574 Y2 S1 P"ystop"                            ; configure active-high endstop for low end on Y via pin ystop
M574 Z2 S2 

; Z-Probe
M950 S0 C"exp.heater3"    
M558 P9 C"^zprobe.in" H5 F120 T6000 A3           ; set Z probe type to bltouch and the dive height + speeds
G31 P500 X30 Y0 Z0.88    	                   ; set Z probe trigger value, offset and trigger height Z0.77  
M557 X15:180 Y15:200 S20                       ; define mesh grid

; Heaters
M308 S0 P"bedtemp" Y"thermistor" T100000 B4138 ; configure sensor 0 as thermistor on pin bedtemp
M950 H0 C"bedheat" T0                          ; create bed heater output on bedheat and map it to sensor 0
M307 H0 B1 S1.00                               ; enable bang-bang mode for the bed heater and set PWM limit
M140 H0                                        ; map heated bed to heater 0
M143 H0 S100                                   ; set temperature limit for heater 0 to 100C
M308 S1 P"e0temp" Y"thermistor" T100000 B4138  ; configure sensor 1 as thermistor on pin e0temp
M950 H1 C"e0heat" T1                           ; create nozzle heater output on e0heat and map it to sensor 1
M307 H1 B0 S1.00                               ; disable bang-bang mode for heater  and set PWM limit

; Fans
M950 F0 C"fan0" Q500                           ; create fan 0 on pin fan0 and set its frequency
M106 P0 S0 H-1                                 ; set fan 0 value. Thermostatic control is turned off
M950 F1 C"fan1" Q500                           ; create fan 1 on pin fan1 and set its frequency
M106 P1 S1 H1 T45                              ; set fan 1 value. Thermostatic control is turned on

; Tools
M563 P0 D0 H1 F0                        ; define tool 0
G10 P0 X0 Y0 Z0                         ; set tool 0 axis offsets
G10 P0 R0 S0                            ; set initial tool 0 active and standby temperatures to 0C

; Custom settings
M572 D0 S0.075			; pressure advance S0.075		@0.6 S0.048

;LED
M950 P2 C"e1heat" ; led

;load data
M501;

; deployprobe.g
; called to deploy a physical Z probe
;
; generated by RepRapFirmware Configuration Tool v3.1.4 on Sun Oct 04 2020 13:36:35 GMT+0200 (czas środkowoeuropejski letni)
; insert codes for deploying the Z probe here
M280 P0 S10 I1  ; deploy BLTouch

Will try doing roll back in free time and see results.

@phaedrux Well, bltouch wires are parallel to hotend heater, but it happens at stage when bed is being heated, while wires from heating bed are routed different way. When I turn on hotend heater (but only manually) it gets worse. Another thing is that before PID starts working (before being close to set temperature), as far as I know, there should be only DC current that shouldn't cause any interferences. Also it's working like that for months without any issues and such problem doesn't occur while printing.

So recently I updated RRF to V3.3 (form 3.1.1) on my Duet 2 WiFi. Since update (first print), without changing configuration files (only changed amount of probing number), probe started randomly deploying itself, sometimes it just deploys , sometimes it deploys and retracts like mad. It was working fine for more than year and a half. It happens only when I setup print and printer is heating, usually it deploys once midheat and stays like that untill z-homing. When I don't start print, but manually set heating (bed and hotend) probe starts delpoying and retracting like mad. So far, probe never deployed while printing.
I'm marking it as firmware thing, since it happend just after update while hardware setup should be fine.

Okay, so I did small research and I think all problems are solved. A bit of information, maybe someone else will also struggle with similar problem.

1. Using buck/boost converters near signal wires (like probes) might induce noise and cause problems
2. If you struggle with unstable printer work/your voltage readings (in DWC) vary under load (even if PSU ratings are higher than requirements of your setup) check your PSU in diffrent position (horizontal/vertical), if it makes diffrence you probably have problem with cold solder joints - if you are familiar with soldering and electronics - redo solder joints in your PSU - if not, better not to do it yourself.

I did small research for my printer, decided to do some tests with opened enclosure of PSU to measure some things and noticed, when I test it laying flat, there is no problem (in printer PSU is mounted vertically), as soon as I mount it back, problem occured again.
Decided to redo all solder joints on PCB (I couldn't find anything that looked really suspicious) with special attention for filtering sections, added also some lead-solder. After that I don't have any problems like before.
I think problem is solved.

@Phaedrux Actually I'd say brand new "no-name" and I feel that might the point.

A bit over month passed and decided to dig this thread again. Problem is even more strange. I replaced fans for 24V but I wanted to reduce PSU fan noise so I put new step-down converter. Printer became quieter, everything worked fine for a while. Today I wanted to run print and I couldn't even home my printer (z probe 1000 error), even though probe was retracted and non-blinking. Decided to make test and remove fan with converter. Printer started working but problem with randomly rectracting probe at Duet reset appears again (like in video in first link). Now I'm messed, is PSU making too much noise to handle by Duet (add additional filtering?) or, exactly or what?

I'm unsure if double posting is fine but I think I've found source of my problem. It's bit unusual. So recently I moved to 24V. Unfortunately I'm curently out of 24V fans so I decided to use buck converter (to run radiator fan) I had and connected it to PSU. Seems that my converter makes enough noise to disturb proper work of probe in such situation like rebooting. Unfortunately I don't have acces to oscilloscope right now to measure noise level, but I think problem is solved.

@PaulHew said in BL Touch deploys on reset:

Take the I1 off of your deploy and retract and try again.

From this
M280 P3 S10 I1
to this
M280 P3 S10

Already tried that without succes :c