Well, I ain't hell bent on understanding what's the matter. Would be nice if I would though.
In the meanwhile, how about using the MCU for protecting the bigger load ?

Q1 = 250V N channel MOSFET
Q2 = logic level N MOSFET
D1 = standard variety 0.5 - 5 kW TVS
D2 = probably not needed, some very low value Zener
MCU has a 330 uF reservoir so it will still operate even on a negative spike; its 5V is protected separately. This ought to work, right ?
LE: actually Q1 will be two back to back MOSFETS