Hi,
Just to help you can try this two things that often fix this issue. First, make sure your Xbox app and Gaming Services are fully updated through the Microsoft Store by going to Library > Get updates, since older builds sometimes ignore saved preferences. Second, try resetting the Xbox app by going to Settings > Apps > Installed apps > Xbox > Advanced options > Reset. This clears any corrupted configuration that might be preventing your chosen startup option from saving.
Just to confirm, does this only happen with the Xbox full-screen experience, or do other apps in your Startup list also reset their settings? And have you made any recent changes to your account permissions or Windows policies that might affect how apps launch at startup?
Kind regards,