Mobs in spawn chunks don't despawn, resulting in no mobs spawning anywhere else

The bug

All creatures (without including villagers and illager raids) are barely spawning in all parts of the game, I am not sure if this is a glitch but I keep going to different new worlds and I keep finding the same issue.

Steps to reproduce

1. Create a new superflat world (for the easiness, but also because mobs are more frequent the lower the altitude is).
2. Once in the game, don't move, and set the time to night.
3. Let the mobs build up, or use a repetitive command block to fill the cap.
4. Once it's done, then fly away from the area, very far away, very very far away.
   → You will notice no more hostile mob, nor animal is spawning the more you travel. That's because they are still loaded in spawn chunks despise no player is nearby.