Hello.

All of my villagers are secured in an area and they are in the proximity of eachother.
This perimeter is 15x15 blocks. There are (was) 23 villagers within. The perimeter was secured with slabs. The Village is under 8 layers of dirt to prevent lightning strikes from ever having a slightest chance of hitting them. That never happened again. Zombie raids never spawned due to slabbing up the entire area + many more layers around this area. I am telling you; I went completely insane about proofing the area for these villagers. Even Iron Golems cannot spawn now. Nothing spawns near the villagers. Nothing.

Every single one is also name-tagged.

So

Why does some of them dissappear when I come back from my journeys, exploring, nether, the end, etc. Places that are REALLY REALLY REALLY FAR AWAY from my village. Why are they even doing anything while I am so far away? Why are they despawning? Why do they even despawn? I don't see any logic behind having this mechanic. Everyone wants their villagers to remain in their village, everything else makes totally no sense. 

Is there perhaps a limit to the amount of mobs that can be named in a said server?
I am really running out of crazy far-fetched ideas as to what is causing this rare but random event to happen where all the sudden one villager is missing. ONE. WHY?