After tortches have vanished from my underground villager hall a zombie managed to spawn and eat them all :'(

I had over 30 of them living in the dormitory but now only 4 remain.

This should have stayed at 30 because turned villager shouldn't despawn since recent updates and as I only play on hard mode to prevent this from happening. The double failsafe has double failed me...

I am not willing to try and reproduce this bug, I lost a lot of progress which costed me months!