When a mob tries to pathfind/flee, it always chooses small waterfalls to get up even if the top of waterfall has surrounding blocks. This causes them to end up drowning.

As of the 1.17 snapshots (21w06a) this doesn't always happen. Sometimes 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.