I have seen this happen a few times in the recent pre-releases. I think it has to do with the randomization of the traps. In 1.1.1, the door to the skeleton king's room is locked and always had a flash trap on it. In 1.2.0, door traps have been randomized, so it's possible that the trap could be a falling door trap that destroys the door. In that case, the skeleton king is free to escape the room.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.