[Edit] I'm revising my initial post to include more correct info
I'm experiencing the same problem and haven't been able to figure out a fix either. Based on your information, you have isolated it to the ARCamera intrinsics matrix. However, this does not seem to be the case from my observation. Here is what I see:
In my case, text and circles draw correctly in the device orientation I start out in, regardless of whether it is portrait or landscape. When I switch orientation, the text and circle becomes stretched (and skewed from some angles). Start in portrait, it is fine in portrait and skews in landscape. Start in landscapee, it is fine in landscape but skews in portrait.
I've looked at the starting ARCamera intrinsics matrix in both orientations on an iPhone (running iOS 11.1 Beta 4) and the matrix is the same when it starts in either portrait or landscape, so it doesn't seem that this could be the answer.
Have you made any progress on this, or does anyone have further insight on how to fix this?