So this bug isn't related to a 4.0 mechanic but I've never had it before and I wasn't able to find a pre existing topic for it.
Two of my characters have their hp go down to 0 during a fight in the Drowned Barrows. They become unkillable and the UI shows them as having either 0 hp or doesn't show anything.
I suspect their hp go negative or the game loses track of their hp. It is not fixed by reloading or resting.
Here's a save before the fight where it happens: https://ufile.io/hdlrj
Here's a save after the fight where it happens: https://ufile.io/v1b52