Thank you for the comment!! Mjoll gives a hint about the code if you talk to him using the Q button while you’re outside the dungeon, although I should probably change it to make him say it inside… that’d make much more sense.
Here's the door code if you'd still like to see it!
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The Insert key is the default key RPG Maker offers in this situation, so it’s not a bug, although I agree that it’s kinda weird.