I have a small scale amount of villagers, only 12 which seems to be the max with the same amount of beds.  All precautions to keep them from dying has been made, all leveled up to level 2.  Half of them or all but a few keep de spawning when I spend a little too much time in the Nether.  I've even come back to find most are gone and the left over villagers will not repopulate because it seems the missing villagers are still existent, just not visible.  Even stranger I have even seen a villager revert back to a child while I am presently watching it do it.  A similar instance of villager de spawning was happening around a year ago and the codebase MCPE-21416 was created to solve the bug.  I do not know if the fix for that particular bug would apply to this bug I am having or if this is a new bug.
Bedrock Edition on iOS 12.5.4