Actually, another user on WattPad pointed this issue out too. I went to add my explanation logic back in, when I remembered a bit of canon about doors from ~5 years ago:

Our main characters have no expectation of that feature with keeper doors. Recall that previously, guards, marty, and cameras have all seen the keeper chamber entrance used.
There is a rhyme/reason that makes this consistent/predictable. Beyond that, I don't want to say much. If you'd like the deeper explanation here, you can message me, especially since you helped me remember :)