TLDR: villagers randomly despawning despite having a secure and functional village setup (has happened three times in a month on the same survival world both on realms and off) 

Detailed explanation:
I have a three story village set up in survival (16 villagers per floor). The first floor contains random villagers (e.g., farmer, cleric etc) and the top two floors contain librarians. On all floors some villagers have name tags, for example a librarian with a mending book has been named mending, and some do not.

All three floors have 16 beds, 16 workstations, lighting, a bell, iron doors with bottoms at the exit points, etc etc as per the normal village setup. I have cats spawning and 6 iron golems locked up just outside the village entrances. If I kill the golems new ones spawn. So I believe it is safe to say I have set up a secure and functional trading village. 

Today I logged into my survival world and numerous villagers (both named and not) have despawned. This is the third time this has happened and I have had to re-breed to get back important villagers. We paid for realms to see if that would help stop the issue and it has not. Please help... I am tired of spending all my time rebreeding villagers.