"must block" plus "menace" bug observed

Hello,

I was just playing a commander game and observed a bug. One player had the card "Grand Melee" in play which says all creatures must block if able. Another opponent attacked me with a creature that had the "Menace" ability. I had only one untapped creature to block with. I was prompted to "select blockers". If I tried hitting done without selecting blockers, a pop-up window appeared that said my creature must block. If I tried blocking the attacker, it obviously removed the blocker due to menace as you can see in the screenshot attached. The game was thereby stuck and I conceded.

Thanks!