When a player leaves the game, the mobs around them don't despawn even if the chunks are loaded.

How to reproduce:
Player A stands in the overworld away from the spawn chunks in a place where no mobs can spawn, such as on a block high in the sky

Player B stands in the spawn chunks and mobs spawn around them.

Wait for the mobcap to be filled by player B, then player B disconnects

player A can now go through the world and see no mobs spawning, since the mobs in the spawn chunks are loaded but not ticked they don't despawn and player B will not see a mob

This has negative effects in multiplayer servers, if a player disconnects in the spawn chunks nobody will see mobs until someone visits the spawn chunks.