not functioning mob spawning

Hello Mojang Team,
i started a new server lately and everything seemed quite normal. But since a few hours, there are no hostile mobs spawning anymore. nowhere. i already experienced this several months ago on my 1.16 map. the problem is the same, besides funtioning phantom spawning on my current map.
due to this "bug" i cant build any mob farms, which makes it impossible to farm gunpowder. that kinda takes away all motivation what so ever. asking people over reddit didnt help either.
i hope you can help me.
some informations about my configurations: 1.18 experimental (cant be the reason, because problem preexisted on 1.16 already), difficulty= normal (switching between these didnt help),
simulation distance = 12, render distance = 48 (cant be the problem, since my farm works fine on a third map with same configurations), everything else is standard

Best regards,

Vincent König