Seeing several Shima Enaga at almost bedrock. The biome is a Snowy Plains, but it seems like the mod doesn't check for height.

Log: https://mclo.gs/nJh2qFT

Hmm, I tested this a bit more and I wonder if some other mod could be causing it...
In SpawnHandler you guys set Red Pandas to follow the MOTION_BLOCKING_NO_LEAVES heightmap, but sure enough, I saw one in a cave underneath a jungle.

Ok, I think I may have found the culprit: Respawning Animals. With the mod disabled, I'm not getting any animals spawning underground. Sorry for bothering!