Secondary multiworld worlds (normal type) cannot spawn hostile mobs when the difficulty of the primary world is set to Peaceful, regardless of `server.properties`, in worlds created with Multiverse-Core > 5.

Reproducible with Multiverse-Core >= 5.0, does not occur with MV 4.x.

The newly created world seems to be created with the allowMonsters SpawnFlag is set to false.

MV5 uses Bukkit's SpawnCategory to handle spawning. https://github.com/Multiverse/Multiverse-Core/issues/1469#issuecomment-2837383203

Verified with Spigot 1.21.5 and 1.21.8.

This affects natural spawning, spawners, spawn eggs, and `/summon`.

Reproduction Steps

1. Initialise a fresh Spigot server with all-default configurations (including Easy difficulty in `server.properties`)
2. Add Multiverse-Core 5.0 or higher (restart server)
3. Using MV, set the primary world's difficulty to peaceful
   • /mv modify world set difficulty peaceful
4. Notice that when a mob egg (Zombie) is used in the default world, the mob appears for a split second before being removed (vanilla behaviour for Peaceful).
5. Create a new `normal` type world and teleport to it
   • /mv create test normal
   • /mv tp test
6. Attempt to spawn mobs (Zombie) in any fashion in the new world. Notice they don't even appear for a split second.
7. Explicitly enable monster spawning in the new world.
   1. /mv entity-spawn-config modify test monster set spawn true
8. Repeat step 6.
9. Restart the server
10. Mob spawning in the secondary world may work now. In my long-running world, they don't, but in a brand new world they sometimes do.
11. If mob spawning still fails in the secondary world, switch back to the primary world, switch it to a non-Peaceful difficulty using MV, and restart the server. Mob spawning should now work in the secondary world.