Analyzing a game I got to a position where with Black to move the pieces were frozen. I copied the FEN to another analysis board, and the pieces weren't frozen but they snapped back to their originating squares so still couldn't be moved. This is the position:
K7/8/1k2bn2/8/8/8/8/8 b - - 100 162
Both scenarios occurred in Chrome 71.0.3578.98 and the second one in Firefox 64.0.2 (64-bit). I'm using Windows 10 1803 build 17134.523.