I was fighting some ghosts.

One of them cast a spell on Eder and he became hostile to the group.

While Eder was dominated, Grieving Mother cast the lv. 6 spell, that damages and knockdowns all enemies near the target.

Eder was damaged and knockdowned by the spell.

I killed all ghosts and combat ended.

Eder was still dominated and prone at this moment.

After combat ended, Eder became friendly again.

But he remained in prone position.

He was able to move and loot the ghosts in prone position.

I saved the game and when I loaded it he was standing normal.

 

I had a similar situation, my PC was a druid firing a blunderbuss. I fought in the Easterwood graveyard against the rogue Eothas priest party. One of their spells knocked my PC druid prone, I think he was reloading at the time.