Heater faults not behaving as expected - wrong heater number

[achrn]

The 3.4 release notes say "To handle the event, RRF runs macro file heater-fault.g without pausing the print first. The heater number is passed as param.D along with some other parameters." but I'm not getting the heater number - regardless of what heater faults, param.D is zero (and also the error message says it's heater 0 that has faulted).

my heater-fault.g is currently just

echo "heater fault"
echo {param.D}

I have tool 0 defined using heater 1 (config.g contains M563 P0 S"Nozzle" D0 H1 F0) and if I set a temperature with no power to the heater element with G10 P0 S215 (or set a temperature in DWC) the heater generates a heater fault (as expected), but param.D appears to always be zero:

Similarly, I have a chamber heater on heater 2 (M141 H2 in config.g) and provoking a fault by setting a temperature without power connected with M141 S99 gets me:

It's
Board: Duet 3 MB6HC (MB6HC)
DSF Version: 3.4-rc2
Firmware: RepRapFirmware for Duet 3 MB6HC 3.4.0rc2 (2022-02-22)
Attached SBC (pi4)