When a village reaches level 15 or so, and the area in which they are in is unloaded, each villager has a chance to despawn, as if they never existed at all. However, baby villagers do not seem to despawn, and after hours of testing, the same can be said for nametagged mobs. They will despawn under the same conditions, however, there must be a significant amount of other mobs in their vicinity for this to occur. I haven’t tested this on any prereleases, so they may be fixed there. This only occurs on multiplayer worlds, so I’m guessing it’s a ‘save mobs that will despawn and accidentally ‘clean up’ the unintended mobs while saving thing’