It never happens in Single Player or in Multiplayer Editor environment.
So only noticed through clients connected to Dedicated server.
It can happen at the beginning of the mission or all of a sudden in the middle of the mission.
I am suspecting that it occurs on dynamically created units but not confirmed yet.
No errors that I can spot in the server's or clients .rpt file, sorry.
I have not run into this error on the Bohemia missions yet . More testing is needed, on that.

I don't want to jump into premature conclusions...but since BIS made me aware of all the "undefined variables" and I have promptly since then managed to fix them...I have NOT seen the AI ragdoll fail :)

I think I have found the problem.
Execution of either [player] call BIS_fnc_cameraOLD; or [player] call BIS_fnc_camera; through a radio trigger causes the Ragdoll effect to not execute.
An AI killed, will drop to the ground immediately without the ragdoll transition.
This is only noticed on a client connected to a dedicated Server.
I can now reproduce it :)

Not a big thing since either cameras function will be disabled after debugging of the mission.

I have notice this in one of my missions.
Don't know what caused it, I'm just letting you know I get it too :-)

Excellent search mate and well defined problem. I guess I have it once but cannot confirm because could be due to lag.