That door only opens for the bad ending. The good ending takes place in the backyard, so unless you are also sure the backdoor wasn't open (there's also a red light that comes on outside to signal/lure you there), then it may be fine. The doors get opened the moment the ending is determined, and nothing else has any interaction with those doors codewise, so I can't think of any way that a bug there would be possible. Thanks for playing though, hope this helps!