I, unfortunately, overwrote my saves. However, I got this reproduce consistently by hitting the next turn button then an AI would attack while processing the next turn. Subsequently since they attacked they get the first move and the game loop seems stuck between unit movement and battle movement I think? I do not have evidence to support this claim as my vision of the map is typically not enough to see most AI units move around.

As a workaround, quick load your last save (F9 by default). Then check for pending battles before pressing the next turn button. You would think that the next turn check would catch them, but it seems that our fingers are a little too fast for the event loop here.