Hi, JumpiestClamp80
Welcome to Microsoft Community.
We understand that you are having issues with FPS games not locking the mouse in place if you join the party through the Xbox Game bar. I note that you have found an alternative solution to joining the party via the Xbox app, and I'll mark your reply as “Answer” to serve as a reference for other users.-Reason for the problem:This is because the Xbox Game bar appears as a control menu at the forefront of the system UI, similar to if you had pressed Windows+Tab or switched to another window while playing a game, at which point the app's focus would be locked to the Xbox Game bar instead of the app, which often leads to problems.As this is a design logic phenomenon, it is recommended that you use the Xbox app to access the party alternative for now, and then provide feedback on this phenomenon you are experiencing to the Microsoft development team via the Feedback Hub, who will review submissions or fixes on a regular basis.Once submitted, please share the feedback link here as this will allow others on the forum (including me) to vote for your submission. More votes will increase the attention and priority of the development team.Thank you for contributing to Microsoft products! If you still have questions, feel free to reach out to us.
Best Regards
Nicholas.Z - MSFT | Microsoft Community Support Specialist