It could be related to the ragdoll settings that you are using. I still haven't mastered these settings but if you do a search on the Unity forum I've found various recommendations. The Unity's ragdoll wizard doesn't work the best with all character portions.
I am now having the same issue. The ragdoll bounces around 1/2 way sunk in the floor. Also the AI char floats up. You can see the collider floating upwards in the editor, and not a little like 10 meters.
Ragdolls can be tricky to get the right values for but when I was creating Nolan's ragdoll I used the ragdoll wizard and then adjusted each collider to ensure there weren't any overlaps.