Placed a platypus inside an enclosure and named it. After closing the game and reopening later, noticed platypus had despawned.
Placed multiple platypus, all named, inside a completely boxed-in enclosure with water. Used barrier blocks to fully close enclosure. Went to build something else and then returned and they had all despawned. This was done without closing the game.

Edit for clarity: Using Forge 43.1.1. Minecraft 1.19.2. Alex's Mobs 1.19.5. Running on 2020 M1 Macbook Air (not sure if this is relevant).