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.

This isn't a bug. The FEN clearly shows that a draw had been reached by the 50 move rule. The 100 in the FEN is the "halfmove clock" that reached 50 moves (100 plies/half-moves):
www.chessprogramming.org/Forsyth-Edwards_Notation#HalfmoveClock
You can reset the halfmove clock by setting it to 0.

OTB, the arbiter must (FIDE; "may" in USCF afaik) declare the game drawn after 75 moves, but after 50 either player can make a draw claim (both as per rule, if no pawn movement or capture occurred). Not entirely sure why there's a discrepancy.

The history of the 50 move rule is pretty interesting. Makes a good googling binge, if you're curious about it.

Thanks, that was helpful. I should learn to read FEN. It's still a bit curious that the pieces were frozen originally and then not in another analysis board, but it's trivial, I guess.

I read Wikipedia's history of the rule. It's certainly convoluted. Chess history generally is an incredibly rich subject.