I discovered a bug during the final boss fight, wherein using a damaging parry on the boss and kettle simultaneously and dying at that same moment will result in a murder-beam-esque hitbox remaining on the kettle, which will eventually kill the king. This results in the ending cutscene dialogue playing out, but you respawning at the checkpoint before the boss. For some reason, this spawns an impassable door similar to the locked one from earlier, softlocking the game as far as I can tell.