@Herve_Smith I don't have an answer to your problem - not experiencing it myself but here is a hack that might work. Personally, I don't use M codes to wait for heaters for a number of reasons which have no bearing on your problem but instead use this format
while sensors.analog[1].lastReading < someNumber
G4 S5
So maybe you could cobble together a hack whereby you set your temperature to be say 5 deg above what you really want, then use that while loop to wait for the actual desired temperature (i,e 5 degrees less tha what you've just asked for) and when it reaches it, set the temperature to your desired temperature and continue. e.g.
M568 P0 R205 S205
while sensors.analog[1].lastReading <200
G4 S5
M568 P0 R200 S200.
Or something along those lines