I noticed after a total reinstall on a new game, with all my old mods, on patch 1.8 with all 3 dlcs(nothing has changed), npcs who sit down on stools in the inn always sink into the stool.
Any idea what is causing the problem?

Just regular stools or the taller ones that are usually at the bar in an inn? I've seen it happen where they sit down and they sink to the regular sitting height. Also if you kill an NPC while they are on one they will most likely get jammed underneath it. But I think that's unrelated.

Sometimes I see it on the inns, and I have all the 3 DLC, plus a nice mod quantity.

I wonder what causes it.