I’ve had the Hidden Forge boss freeze the game about 10 times now. It happens after he does his breath attack (occasionally, not every time). No damage is registered and the game does not bring up my characters menu, it just sits there. I would estimate that this happens to me about once every five times I face the boss. Every time it has happened I was in room 6 (possible that this is not room specific as I clear room 3 before he has a chance to breath and I never go to room 9). I have to force quit the game and restart it to be able to continue (means I have to start the fight new every time). I am 95% sure I have had this bug happen to me on two separate devices.
Edit: appears that this bug has been around for a while