Sounds like this might be related to that other thread (link escapes me at the moment) about ac getting screwed up when dragging to the vtt? I've had it happen to me with a couple variations: 1) only the token bar value is affected/blank, and 2) the content of the ac field in the npc sheet has disappeared. For my case, these were compendium creatures as I am not running modules. I don't have repro steps, as this this seemingly happens at random for me (happened once in my session last Friday, for example).