Indeed, this issue happened to me as well with the following npcs :
- Rolan (sometimes he leaves alone)
- Lia and Cal (always stay sitted)
- Kaldani
- Liam (Tortured man at Goblin Camp)
- Benryn (Burning tavern at Waukeen's rest) : after saving him from the fire, he stays on the floor in the fire x) Using Speak with Deads on his partner freed him from this bug if I remember correctly.