Some Peaceful Mobs Despawn While Named Sometimes

Named peaceful mobs will despawn sometimes. I say only sometimes because I have them in an open area where mobs cannot kill them, when I leave them unattended and come back, there usually always there, but sometimes I come back and there gone. Sometimes they'll even straight up disappear when I'm within viewing distance of them still.

For example, my foxes have been disappearing even when named in these situations. The only thing I have a somewhat understanding of, is even when named, I noticed they won't despawn when kept in an area where they can't roam to far. I have a couple houses, in house 1 which is smaller, they won't despawn from what I've seen so far. However In house 2 which is way bigger, it seems they have a chance to despawn. I understand mobs despawing in cases where there not kept within a certain amount of room and enclosed, but the foxes or any other peaceful mob I've had were named. Which I thought name tags were put into the game so they didn't have to be kept in a certain amount of block space. It was either changed or a bug like I'm thinking.