The only thing i can think of that might be causing this issue, is the same thing that was patched before, when an entity is standing between 2 or more chunks and has a small chance to be deleted if the chunks do not load at the same time.
similarly, i also had a tamed fox, as tamed as one can get a fox, and it too despawned. As well as a few cats and parrots also despawned even being tamed.
here is an example, i had at one point a skeleton horse, mule and donkey, all of which had nametags and were in small enclosed spaces, small enough to prevent despawning, however they still despawned when they were tamed domestic animals, this was not an issue untill after the nether update.
I experience the same issue with my villagers on nintendo switch...despite having fully protected it with light, walls and golems...on top of that, having each villager with a name tag
in addition, this could include: foxes, panda's parrots, cats and wolves. Cows, Sheep, and pigs seem unaffected by the despawning.