The title and the video explain everything. This happened while I was modeling a holder for a plastic bin, and is easy to replicate. I also updated and restarted to see if it happened to be fixed, and it wasn't.
In words, if you put a tube with a hole through two blocks, then delete the outside face of the tube that is between the two blocks, the inside geometry will stay and be visible, and probably cause other mathematical issues further on.
I ended up with only the inside face of the tube between the two walls. The walls themselves went flat, while the inside face intersects through them, still trying to be the one inside face that the tube on the left and tube on the right shared prior to the delete. The video first shows it properly being split, but when the offset plane is added, it causes bugs.
Edit to my previous response: Apologies, misread your message. What I wanted to end up with was this:
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