Hey germ1389,
I have just tested this internally and have been able to see what you have been experiencing. This is working as intended though. If you add tilt to either MoveRight or MoveForward, It is only going to stay along those lines. You would have to set it up in blueprints to correctly reference the tilt of the mobile device.
Think of the mobile device as your keyboard keys. You press ‘W’ to move forward and ‘A’ to move left. If you move them both, you move in a diagonal. This is how tilt is being referenced by adding it to the Axis Mappings. Since it is added to both MoveForward and MoveLeft, this is why the diagonal movement is seen.
Please let me know if you have any additional questions.