If it is in fact the moment you walk in the door, then it absolutely is the architecture. If the architecture didn't have any effect on it, whatever's disrupting the signal would also interfere with the signal outside.
Damn near every school building I've ever been in is a behemoth of brick, concrete, and cinder blocks. Cellular and other radio signals have a hard time penetrating that.
Same for a lot of hospitals, big retail stores, and other similar places.
I work in 911 dispatch, we have caution notes attached to the addresses of a lot of schools, hospitals, various office buildings, etc. in our area that there's poor cell reception or that our responders can't get radio reception inside the buildings, so we know how we can or can't communicate with our units when they're responding to an emergency there. I can guarantee you those places aren't purposely jamming police radios.
I lose my cell signal in parts of several of my local grocery stores, big box retailers, etc. that's just part of being inside of a big concrete and metal box. Why would they even want to interfere with my ability to use my phone?
A lot of these buildings were built before cell phones were even a thing, so reception was not a concern in their design. Even in newer buildings, it's often not a major consideration.
And as others said, jamming a cell signal is a huge no-no from the FCC. If anything, and I doubt they're even doing this much, they have picocells (basically tiny cell towers) in the building that they're turning off at certain times. If they didn't have them, there would still be no signal in those parts of the building.