Event mobs cannot be named, leading to despawning.

I have been attempting to construct a zoo of the currently available spooky mobs and have noticed that while naming them does consume the nametag, the name is not visible. My man piece of evidence that the nametags are not working is the fact that the mobs in question (Pig, Chicken, Skeleton, Creeper, and all other vanilla mobs along with wraith and gullet) despawn within a couple of days. This has resulted in me losing about 5 spooky serums and far too many capture nets. From other players reports, I believe this is also the case for festive mobs. Because of this, I belive the issue stems from something in the code that produces the variants.

Many thanks.