i have been playing with a few others on my own server for a few days now and have built some mob farms there.
The problem is that in the mob farms no mobs spawn when someone else is in the same dimension (not only less spawns there but really nothing) even if the other person is hundreds of blocks away and not in the spawn radius of the farms.
We play in the 1.16 and only have a sleep plugin and Dynmap as plugins We didn't make any changes to the server settings.
Is there a way to fix this problem?
Even if they pillar up to build limit so there are no mobs spawning near them?
If they are in a random place in the wild there are probably more places near them for mobs to spawn then in your mob farm, and if they aren't actively killing mobs the mobs will reach the cap and stop spawning.
You could be right but unless you do some proper testing it's more likely that the mob cap just filled up.