The bug happens in Studio. When using the Emulator to test mobile gameplay, using the Screen Rotation button at the top should change the CurrentScreenOrientation property of PlayerGui to Landscape or Portrait, respectively. This property changes properly live in-game, but doesn’t in Studio which makes testing a nuance.
Go into Studio and turn on the Emulator. Make sure ScreenOrientation is set to Sensor in StarterGui. Test in Play Solo, and look at the player’s PlayerGui’s properties. CurrentScreenOrientation doesn’t change when you click the rotate screen button.
This bug happens only in Studio. The property functions normally in-game.
Video of bug: Watch 2017-07-20 05-33-10 | Streamable
Credit to AlreadyPro for finding this bug, as he’s not a part of the devforum to report it and I’m just doing it for him.