Renamed Zombie Villagers Despawn

Converting a named villager to a zombie villager, they keep the custom name, but after flying away and returning they have despawned

It is as if the game thinks they were an ordinary, non-villager, non-renamed, hostile mob which can despawn.

This has happened 3-4 times now so I know I'm not imagining it. (on survival, in Minecraft Realms, on Java Edition)