In Mayrina's questline we used our passive skills to knock her out because Auntie Ethel used illusions and we did not know which one was the legit Mayrina. So she was knocked out with one health, after winning the battle, we long rested so that Mayrina would wake up. When we came back she was gone and nowhere to be found in or around the house. I feel like this was a bug?
Knocking NPCs out has pretty much never had the desired result. One exception I've noticed is at the windmill. You are actually able to proceed to wake and question the NPC after knocking him out. This is a specific case though and you're told to knock him out so clearly Larian finally tied off that outcome.
