playing on the atm9 TTS modpack
i have been doing a bit of testing, trying to create mob specific farms, creeper ,spider etc. and I'm getting some super weird spawns happening
slimes need 2.1 blocks of Hight, however I'm getting large slimes spawning in gaps as small as 1.5 the large slimes are getting stuck in the roof above them, other mobs such as enderman, zombies, skeletons seem to be working fine, and respecting their relative spawning conditions
spiders also, cannot spawn in a 1 block high gap for some reason

Not sure, the dirt just uses the vanilla NaturalSpawner.isSpawnPositionOk to check if a mob is valid to spawn.

There's definitely something weird going on because slimes are being spawned in walls and being pushed outside of the farm

I have the same issue, had to enclose my grinder in 2-thick walls to stop slimes from glitching through. I'm also having problems with the dreadful dirt not spawning anything in the nether.

Im not sure what to say, we are using the vanilla NaturalSpawner.isSpawnPositionOk with the restrictions from the mob being spawned.

This ignores the mobcap right? Because the issue I'm having in the nether is that right after I place the dirt there will be a couple spawns, then it won't spawn anything anymore, even in a vanilla biome like nether wastes