Aternos server mob spawn problem in 1.18 with normal and hard difficulty

Monsters normally spawn in night but very little.We did mob grinder but don't work.We did 0 light level room and didn't work.When we try same map with singleplayer 1.18 it works normally and mobs spawning everywhere.Why didn't work in aternos ? Monsters spawns nearly every 5 minutes .İn caves mobs spawning normally by the way.

we dont have any plugin.Normal server not bukkit

we tried 1.18 paper bukkit version and mobs spawning normally it seems fixed.İs there any plugin for increase mob spawn higher than normal in bukkit ?

and spigot or paper which one do you suggest ?

we did mob grinder 100 block above from ground for per player 70 mob spawn limit.İt works when we together with my friend but when he leaves and go to ground , mobs dont spawn anymore.Same goes for when i leave and my friend try to mob grinding farm.Looks like not per player 70 spawn limit only two player in one are can 70 spawn mob limit.How can we fix it do you know ?