while playing on an existing world, if friends join locally mobs stop naturally spawning, even if they had name tag. after friends left hostile mobs start spawning but name tagged mobs don't.

this happened on a version before 1.16. Now again after this Update 1.16.201.

this do not affects spawners, they still spawning mobs correctly.

Yes same area, and even if I go to the nether is silence, zero mobs on the nether. I do not think is the mob cap. 

And yes I know tagged hostile mobs don't spawn, the expectation is that they don't de-spawn, but they did so basically all hostile mobs regardless of being tagged or not they disappear and after my friends left the game all other hostile mobs start natural spawning.