Humm, so after you finish the 2nd set of levels (the dark area), you enter the hub again...and the 3rd door opens. Then, when you save, it puts you back in the hub with the 3rd door closed? Or does the 3rd door never open? Just trying to narrow it down :)
Good ideas with the achievements! I hadn't tried tackling them yet, but it is a good next step (especially now that it's on steam).