The old village disappearing act

This has been on going since the nether update my world is 1.5 years old and very established. I have prisons of villagers at 7 different levels underground. Lighting is adequate (never having a mob spawn inside) and I roughly had about 70 villagers beds tables bells the works. Randomly (seemingly at bed time) my villagers would disappear sometime 11 a day. The few I have now less than 30 seem to be holding up unless I breed in which they start disappearing again. It's really put a stink on the game spending all these resources to upgrade villagers and have the poof away. Any ideas what's going on?

 

world created survival dec 2019

difficulty Hard