indeed, same happens with google maps navi. I have noticed that some applications (eg. hbo go) don't go back to portrait but there is a black screen appearing for 1 sec.
So the solution was finally found and it works! Go to Settings->Apps->tap the three vertical dots on top right corner->special access->appear on top. And here disable all applications. It works and so far I have not experienced any issues with the Phone. It could be that the problem was caused by one application, but I disabled all. Please let know if it worked for you as well! Cheers