When I am on my friends server,(hosted from aternos.org)there are no hostile mobs spawning(except phantoms) in the Overworld but in the End and Nether dimensions they spawn as they should.

BUT, when there are two people on the server all mobs in the Overworld all hostile mobs spawn as normal and when there's only one player online none spawn.

Yes, the world is on hard mode.

Yes, mobSpawning gamerule is on true.

In single player everything is fine. It's only on the server.