I think I might be able to help here, although my experience relates to the OnTrigger block. I had a problem just about like this and found that it was not an engine bug, but rather an engine quirk. I suspect strongly that Bethesda's internals for OnTrigger may be the same as for On.... other things. :-)
In my case, the symptom was that I had a dungeon cell that worked perfectly for months. Then, suddenly, I would hang the game every time I did a "coc" to that cell for testing. If I walked into it normally, no problem. Finally I found the issue. There is a trigger zone in the cell that looks for a particular creature which is supposed to never enter a certain area, and usually doesn't. But because of some Havok weirdness, occasionally the creature passes through a solid obstacle and gets into that area anyway. No problem....trigger zone, detect that actor, move them to an XMarker outside the forbidden area.
It turns out that if you have a MoveTo of an actor within an OnTrigger block, the MoveTo works as planned but the trigger detection does not stop until the *next* frame after the move. The Havok bug doesn't happen during normal gameplay; it only seems to occur during the cell loading process when it is settling mobile objects. So if the creature was in the forbidden zone exactly when I "coc" into the cell....zap! Hung game!
I beat my head against this for hours, then stumbled upon the note at the bottom of the OnTrigger wiki page. May the Divines bless whoever posted that little note!
Solution: Don't do the MoveTo of an actor directly in the detection block. Instead, set a variable in your object script from zero to one during the hit detection. Then do the MoveTo from the GameMode block, and clear the variable to zero when you do the move. IMPORTANT: Make sure the OnGame block is *before* the OnMagicEffectHit block in your code, so the OnGame code will interpret on the *next* frame.
No guarantees this is the problem, but coders tend to reuse logic once it's tested, and I think there's a good chance Bethesda did that here. Hope this helps!