Can you get the host to rerun the turn, for research purposes? I assume that the Lord of the Wild Hunt is unlikely to strike in the same place. If the bug still shows up, we could rule out LotWH as the cause. However, that does seem to be the main suspect.
We've already moved on to the next turn, but I did save off all the game files so I could run it again. I'm of the same opinion that it is probably the Lord of the Wild Hunt which has caused the problem.
Just completely guessing out of thin air... I'm thinking that there is some kind of attacker/defender flag that is used during seiges. So Pythium is the Attacker cause they kill my PD and siege my fort. But then later the LotWH attacks them *after* the normal combat phase, not before like other magic spells (which I have always thought was really odd!). And I think this late-turn attack flips Pythium to "defender" status or something? Which of course would have no effect except when they were the aggressor in a seige.
I had to change the mod name slightly to Recruitable_Rebalance.dm. That is what the error message asked for, so that is what I renamed the relevant file.