Both my brother and I have a Microsoft account that has been granted access to the beta. I setup my profile first on our computer and played through some of the beta. Once I was done, I signed out of my account and my brother signed into his. When he launches the game, it still has all of my information. I tried reinstalling the game as well as deleting any relevant files I could find in ProgramData, AppData, and Documents. I also tried creating a new user account on my computer for my brother to use his account with, but my data was still there when he launched the game. (Different account should use different gamertag, but it isn't doing that as you can see in these videos)

https://www.youtube.com/watch?v=quDDkBJCHp0

Used another key to get my 3rd account beta access just to show off this bug, different account name, same ingame gamertag

https://www.youtube.com/watch?v=Gj3XNkfJ9-4