(Filing this as an engine bug because it relates to an in-game property being broken, even though the mobile app is involved in the process)
I encountered this problem on my Google Pixel 2 phone while using Google Chrome as the internet browser. When you enter a game by pressing the green play button on the Roblox website on your phone, Roblox will automatically open the Roblox app on your phone and load you into the game. However, upon loading into the game your screen will be forced into portrait mode as seen in the screenshot below which I took on my phone.
You cannot rotate your screen to put your phone into landscape mode, even if the ScreenOrientation property of the StarterGui instance is set to LandscapeSensor. You are stuck in portrait mode. This is the current value I set in the ScreenOrientation property while reproducing this problem:
This seems to only happen if the Roblox app is closed, meaning Roblox has to open it first when you press the green play button. If the Roblox app is already open in the background, you seem to be able to load into the game as normal.
Expected behavior
I expect to be able to rotate my phone into landscape mode if I have ScreenOrientation set to LandscapeSensor, regardless of my method of joining the game.