Yeah, to say that some may mess up is logical the idea that ALL of them tier 1 and 2 and the online notification system can all be wrong is not logical and very highly unlikely.
My guess is the the stamp on the i751 form is only a Part of the denial process due to no interview but not denied in any public way or official way. The n400 was denied properly hence the immediate notification of denial on the uscis website.
Tier 1 said at one point that their is no way it can be denied and both the petitioner and the uscis officers not be able to see it publicly.
Keep in mind: the foia request only sent a copy of the i751 we filed with the denial stamp, no denial letter.