Does it not even show on your profile when viewed on the Xbox website? I still see it there, even if I've also not been seeing it on the console OS since I joined Game Pass Ultimate in Preview. I'd assume your tenure is still being tracked behind the scenes, seemingly the Xbox OS just has to catch up to Ultimate being a thing, which in this respect still feels like a WIP despite the service going public. I'd understand tenure reflecting on the profile not being a priority when implementing GPU earlier this summer.
Xbox Profiles in general could use some spicing up compared to some other platform's user profiles.
Thankfully it updated back and it's actually 15. Lost track of time lol