“he falls into the undermesh.”
I am not sure of the situation this term implies.
Is it referring to the NPC who is stunned and falls deep underground?
If so, I don’t think it has anything to do with the fact that this time it is a corpse, not a fainting body.
I have never seen a person I killed fall deep underground.
Or do corpses (including summoned ones) also sometimes fall deep underground?
Either way, the method you mentioned was tried before you mentioned it and right after the problem occurred.
And it did not solve the problem.