With server Render Distance 5 (and probably lower and maybe also lower up to Distance 10), mobs don't despawn when a player leaves the area mobs were spawned in.

They seem to get unloaded before they can get despawned, and are saved in the chunk until a player comes back.

This results in unwanted mob switches around the spawn area, since the mobs are always loaded in the forceloaded chunks and lazy chunks, which count towards the mob cap and stop monsters from randomly spawning elsewhere.