Bed temperature command going to 0 °C with no obvious reason when print is started
I tried, by instrumenting code, to know where/when the temp bed is modified.

I found that it is called in the gcode M81 when starting the print.

Wait M81? isn't it M80 to switch on the PSU?

What I did in fact was both wiring this up side down and mixing M80 with M81.

But what I didn't know is that M80 and M81 are not strictly opposite functions.

Indeed, M81 puts the PS_ON signal to PS_ASLEEP != PS_AWAKE but also disable all heaters.
I couldn't see it without serial trace because the extruder temp is set back after the M81, but not the bed temp.

Problem solved.

Hope this can help people in the future to spend less time than me on that simple mistake.

Note: What remains a mystery, though, is: why was it working before on previous prints and then, suddenly, broke?!!