That doesn’t seem to be it either. I’ve completed this fight a number of times in different playthroughs, including ones when I didn’t harm the masks during the fight and they still remained hostile after Ethel died, whereas in other games when I thought I’d done exactly the same thing they got an attitude adjustment at the end of the fight and turned neutral. And recall, it’s not that they remain hostile permanently, as I can run away and come back and they’re no longer hostile and will talk to me, though only one has a mind left.
Given the behaviour is so unpredictable from a user perspective I still think it probably is a bug, but if it’s intended and there is some pattern that I’ve not spotted, then I think it’s poor design and should be changed. I can metagame and run away and come back to talk to the freed mask, but why on earth would a new player discover this? I suspect only people replaying the game who have had the masks turn neutral immediately in another playthrough would even try, so it feels that all that having the masks sometimes stay hostile for longer is doing is cutting some users off from fun content for no good reason I can see!
Hmmm that definitely seems to be a nasty bug, yes. I think it's intended that you meet Auntie Ethel again even if you've already killed her once. The statements of the masks suggest that. Probably you also would meet the enslaved masks again. (Later, with a higher level, you could use spells like Remove Curse on them to free them for good, because that's not possible in the low level area! After all, you need this spell four times and that would be easier accessible in the advanced game...) But as long as you've eliminated Ethel, the masks should remain peaceful in any case.
