My username was changed to GameDvrDisable for some reason

Definitely check if you have any messages from xbox live. (Bottom tab of the messages menu on the console)

If you have no messages I'd suggest getting un contact with xbox support.

Hello SojaMilch,

Thank you for posting on the Xbox forums. We understand that your gamertag was changed without your knowledge. The first thing you should check is if the gamertag was changed by the enforcement team. Did you receive any messages from the Xbox Enforcement
Team regarding your gamertag? We suggest checking your
enforcement history. If you do not wish to retain this gamertag, you can
change it.

Either your old gamertag was changed for violating the live code of conduct, or algume accessed your account and made the switch.

Hello SojaMilch,

Thanks for coming back. I don't think this is a forced gamertag change. Usually when this happens, you get a generic Word+word+numbers gamertag (LuckyFish##### for example).

The one you have doesn't seem to go by the blueprints when it comes to forced gamertag changes.

Did you first notice this when signing into a new device, or redownloading your profile? If so, this means you signed into the wrong account. You can try contacting support to see if they have the ability to see your past gamertags :
Customer Support

Might change your password as well