[ano]

I think there's some strategic reason that we just don't know. Obviously I don't have that old testbed game but it is quite easy to reproduce with debug mod.
Unfortunately I don't have time for it now. Btw, the next turn after the one I wrote this post I repeated the orders and situation was the same: UM was not cast.
One of the spell that definitely causes more than 200 fatigue and is cast always is Fire Storm. It is fatigue 500 and requires F5. ArchDevil without boosters scripted to "Phoenix Power, Fire Storm" never fails.