Hi, i have a identical problem, i just found this post doing a search online.
I think this has to do with the placement of the back camera, it sure seems like it in my case, anyway. I have Nexus 10 and the camera is on the side, not at the top. Landscape is a default orientation for this tablet. I've searched high and low and found no solution to this problem.
I've tried calibrating the sensors, but everything seems fine. I've also tried using apps that force a particular orientation - and it might work well while walking on the map, as soon as the AR stuff kicks in, the tablet is forced into a portrait while the camera remains in its default (landscape) orientation, no matter what settings i use or what app I use to try to override it somehow.
It looks like this issue is with the app, not the tablet. Let's hope Niantic resolves it in future updates tho.