Not sure how to recreate it, but there's a bug where quite the amount of mobs are thought by the game to be in the world currently. As in, in the monster mob cap in the F3 debug menu it says something like M: 4, meaning there's 4 hostile mobs hogging the mob cap when the player is in places where hostile mobs couldn't possibly be, as in way above the build limit (Y=500 and higher).

Sometimes this value is over 40, meaning the game thinks there's 40 hostile mobs hogging the mob cap which isn't the case.

This is obviously clear in places like hostile mob farms where the amount of mobs spawning and arriving to the killing chamber actually is affected by this number: if it says "M: 42" in the F3 debug menu, then there's actually going to be like 30 or less mobs on the farm.

This number also stays the same after switching to Peaceful mode.

Edit: screenshot attached. Also, the "ghost mobs" appear to be removed from the mob cap when exploring already existing chunks. As in, when switching to Peaceful mode and exploring the chunks where monsters spawned a lot before, the amount of hostile mobs decreases, goes back to M: 0.

Another important note: i usually play with the render distance of 4.