Ants are able to hit through cliffs...

[pawnking2]

I decided to raid an anthill and to get away, I jumped down a cliff. For some reason, I was still able to take damage from ants while they were at the top of the cliff.

Cliff:

[ApocalypsePlease]

They probably only calculate distance in the 2d plane not accounting for height. The game's combat system probably views them as next to you with you at the bottom and them at the top.

If you move further away from the cliff this should stop it from happening.

[VDZ]

Rather, the game does not seem to properly recognize the height difference right after jumping/climbing; most likely it only has one known height for the entire tile, not accounting for it changing halfway through the tile. This also leads to pathfinding not working properly; if you try to pick a forageable right after jumping/climbing, it refuses to move, as if you were still at the wrong height. You need to move a tiny bit away from the cliff to make it work again.

[jorb]

Rather, the game does not seem to properly recognize the height difference right after jumping/climbing

Indeed. Pseudo-intentional.