Hi, In my opinion, structure should not collapse without any force acted on it except ambient air if it could be build when I set pressure to all surfaces, the results looks like there's other external force collapsing the structure, and therefore the structure deformed. Picture below is the simple geometry before calculating, and the next one is after calculating.

If I am correct, then this means that the structure would suddenly deform after entering ambient air, and that's the reason I felt the result is weird.
Hope this answer your question.

Thanks Eric