Questionable DeSpawning

I have a game mode in which players can select to get spawn eggs. They use the spawn eggs in order to defend an objective. I have been having a problem with skeletons and wither skeletons despawning but the other mobs included in the game such as the witch, shulker, and zombie are fine. I have run this on both a server and single player and they both result in the same thing happening. I call it despawning but I'm not quite sure what's happening. The game has players switch from the arena to a private room and back. Whenever they return from the private room, the skeletons sometimes despawn. It sounds like that's all it is. But I feel like it shouldn't be mob specific. And why would they despawn if I summoned them?