No hostile mobs and  are spawning in any of the newly created, or imported 1.18-pre5 worlds.

Plugins are not part of this problem, as it occurs with and without them. This reports is made with no plugins, which leads me to believe this is purely a spigot problem. Vanilla minecraft 1.17.1 or spigot 1.17.1 doesn't produce any kind of problems like this.
Only mob occurence I personally found was the sudden spawning of 1 phantom at night. This was random and was encountered only once.

Later I installed Essentials plugin to check the /killall monsters command. In all worlds it deletes 1-3 creatures, then deletes nothing till the server is restarted.