So i finally saw this behavior happen in real time. I was holding the phone in portrait mode reading a text in the Messages app. An email came in and displayed the brief notification at the top of the screen, the phone vibrated, and the orientation of my text immediately switched to landscape and stayed there even though the phone was still held in portrait orientation.

I had disabled the debug logging after running it for several days with no incident so didn't capture it. Might play around with this when i have time to see if i can repro on demand, but curious if anyone else sees this same thing. If so i can post to the bug.