Mob spawning / despawning appears to be broken and has been since 1.13. This may be related to the return of MC-69752, as well as MC-134969 and MC-134506.

To test: you can make a command block to kill any and all entities in a radius of 128 or greater and then teleport into the air at a distance grater than 128 blocks from the ground. Anything beyond that, mobs should not spawn and if they are spawned in, they should immediately despawn. However, the command block will continue to kill any and all mobs until MC-134506 takes place, requiring new chunks to load/unload before more entities are allowed to spawn in. The, even moving back to the initial spot will still yield a constant flow of mobs being killed from the command block output in chat.

I believe this bug is the root cause of a lot of the mob spawn issues many players are facing. I have made a video showing exactly this scenario.