OK, then I think I understand what happened. The 12V Fan was cooling the MCUs, but since the temperature measurement is done inside the MCU itself, the temperature of Mosfets is not known.
In general it is always better to put air flow on the Mosfets, not MCU, because Mosfets produce heat.
I think this explains the behavior when MCU was reading about 90C but ESC started behaving strangely -- the Mosfets were likely much hotter and got de-soldered..
Alex