Hey, I don't really know where to report bugs on this discord so I'll put it here. I keep running into the same issue over and over ever since I started playing Forge. It's hard to isolate how it happens but it happens multiple times per day for me.
I usually play pretty fast, skipping through phases with the enter key. Sometimes - and I honestly don't know what causes it - when I attack and skip through combat too fast the line indicating the creature attacking my opponent disappears. Once that happens it's usually game over. CPU will not pass after combat damage step.
Doesn't matter if you concede or wait until your hair grows down to your knees, the game will not continue. You need to shut the game down completely.
EDIT: This happens in draft and regular adventure mode battles.