Has happened twice now.
If you die to the boss and use a key to respawn, it will sometimes make it so your D key to swap timelines is simply not there. Even when I’ve then beaten the boss, when you get to the reward area to imprint you simply cannot go to the older timeline and thus can’t drop an item into the forge. Completely negates the whole purpose of your run, and you’re out 2 keys just like that.