According to Stefan Trethan the connections on the JBC T245 tips are different than previously stated.
According to his findings, on this tips the thermocouple is between the outer sleeve and the inner contact, so if the inner contact is connected to the pulsed (PWM) 24v rail the iron tip will have approximately the same voltage (difference will be the TC voltage).
This will make things go boom if ground is connected to chassis ground, as suggested in order to use the stand sleep function. Even if this isn’t used, having the iron tip with such a large voltage is not ideal either.
The workaround Stefan found is to separate the TC reading from the heater element supply. This is achieved by cutting the trace from the output of the mosfet (as shown on the picture below. The inner contact of the tip is then connected to terminal 4 of the plug (the one connected to C7 and R8), the middle contact of the tip is connected to the output of the mosfet, and the outer sleeve is connected to ground.
Thanks Stefan for finding, reporting and fixing the issue!