I am also experiencing this issue although I do not need to involve the combo box.
If I pinch zoom in one orientation, then rotate the device to the other orientation, the display only covers a part of the screen with a white covering the rest of the area. Then if I change back to the original orientation, the white area persists.
Thank you for your report, Phamus! I'm glad I'm not alone with this! I hope the MS staff will get back to us anytime soon about it. I don't know why they have remained silent for so long. The issue is really critical when it comes to end user roll-out since it risks entries of users being made to be lost.