Sigh…why is this thing the way it is.
There’s a chance M503 won’t return the correct values - the PID values in the controller will be returned instead of the PID values in the toolhead. There’s a function defined to read the PID values out of the toolhead but it’s never used.
M301 looks like it sets both the toolhead and controller EEPROM to the same value, but there’s really no way to know what’s in the toolhead at the moment.