I've encountered this a few times as well, usually when I had placed an obscene amount of barrels and fired them off. It sorted itself by me going to make coffee, then coming back to a fully functioning game. I suspect it might just be the engine running a little low on memory in situations where a lot is happening.
Interesting... similar experience here - I encountered it during the fight with a certain very large spider where I destroyed some webs with explosives and multiple creatures(friend + foe) fell as a result. I originally thought it was from trying to end turn with a character prone, but the issue reoccurred several rounds later when everyone was on their feet.
Interestingly in my case I could save mid-fight, reload the game I just saved, and then the turn would end properly. But I had this happen twice in teh same fight(the first time, and again several rounds later).