Opening the MM locks the screen to landscape, this is normal. However if the screen is already landscape, sometimes it will flip the screen to the opposite landscape when opening the MM, now the game is upside down.

Often when playing the game in landscape, the screen will randomly rotate to portrait, then back to landscape, then portrait, in an endless loop despite not moving the device at all. This can be somewhat remedied by locking the screen to landscape (if you can manage to press it while your screen is rotating itself back and forth). This seems like it might be triggered by walking into close proximity to another player in game, though that is somewhat circumstantial.

If you lock the screen to landscape in the OS, then open and close the MM, the game will rotate to portrait and stick there even though the OS is still locked landscape.

All of these rotation bugs combined make the game almost unplayable on mobile, unless you are very determined. I have to lock my screen and I become afraid to touch the MM at all for fear it will rotate itself some random orientation.

Also the non-game app ui locks itself to rightside up portrait, which after all the wrestling with the orientation in-game rotating your device every which way, you quit and are returned to an app that is now upside down or sideways, which feels like the final punchline.