On the Oneplus X (Model number: ONE E1005), pressing the back key opens the chat window as opposed to the game menu. Pressing the back button again does not do anything, making the only way to leave the chat window to be closing the app. I am not aware that this occurs on any other models.

EDIT:
The actual chat button DOES work correctly, but pressing the back button opens a chat window that seems to be only usable if you have a controller. Oddly enough the player can get to the pause menu if they switch out then back in to the app.