Summary
After installing MythicMobs on my server, my players complained that their creeper farms were no longer spawning any creepers. I tested by installing and removing the plugin several times and observed spawning behavior return to normal when MythicMobs was uninstalled. I have provided a schematic of the farm in this report and a command block that will highlight creepers in the farm for ease of viewing. [Legacy attachment omitted]
Version Tested: MythicMobs v4.10.1
Server Version: Paper 1.16.4
Steps to reproduce
Step 1: Install WorldEdit
Step 2: Place the included creeperfarm.schem into plugins/worldedit/schematics
Step 3: Load and paste the schematic twice, once above ground at around y82, and again underground at around y10. Use '//paste -e' to include entities while pasting (cats are used in this farm)
Step 4: Enable the lever on the repeating command block on one or both of the farms to enable highlighting creepers inside the farm.
Step 5: Run the /butcher command to clear spawned mobs
Step 6: Observe the spawning behavior of both farms, creepers should spawn as normal in the farms.
Step 7: Shut down the server and install MythicMobs
Step 8: Start the server and run another /butcher command to clear spawned mobs.
Step 9: Observe the spawning behavior in both farms again. You will notice that no creepers spawn in the above ground farm at all, and very rarely in the underground farm which is unexpected behaviour.
If creepers are still spawning with MythicMobs installed, unload the chunk by moving away and reload the chunk, cycle between spectator and creative, and wait 2 minutes. You should notice spawn rates become heavily reduced and eventually the creepers stop spawning.
Current behavior
Creepers no longer spawn in creeper farms built above ground, and very rarely/never in underground farms.
Intended correct behavior
Creeper farms should function as they do normally when MythicMobs is not installed.
Proposed fixes
My guess is that its likely an issue with random spawning messing with natural spawning algorithms, however I don't use random spawning on my server.