There have been multiple times where a mob spawns or gets in my house and tries to pathfind/ flee, and they always choose the small waterfalls to get up, the thing is, at the top of the waterfalls there are full cobble blocks, and they try to swim up without ever noticing and they always end up drowning.

As of the 1.17 snapshots (21w06a) this doesn't always happen. Somethimes the cat will go to the side of the waterfall and fall down, sometimes it will just not go onto the waterfall at all, but sometimes it will swim up and drown.