Game Pass on mobile still not showing recently added titles after days of them appearing on...

On the website it says "Check out current Quests on your Xbox One in the Game Pass section or on your Game Pass mobile app. New Quests added every day." And that the game pass app shows the quests as well.

I also recently had this problem today. Someone added me, I added them back and now they no longer appear as a Follower, but on their Profile page it shows that I am still in their friend list.

The Game Pass twist on the Home screen of your console, states recently added and leaving soon. Also the Game Pass mobile app has the same information.

They won't become obsolete, the Xbox 360 was still supported for years after the Xbox One launched in terms of both third party games, and dashboard updates. I wouldn't expect too much in the way of brand new titles, but the consoles will still work and
I'd imagine they'll continue adding Xbox One titles to Game Pass for years to come. The plan seems to be that all Xbox One titles will work on the Xbox Series X, so I would be shocked if we don't keep seeing things added to Game Pass.

Where are you checking for completion? In the Microsoft Rewards App? In the mobile Game Pass app? Or is it that the Achievements themselves are not showing up on your profile when you've completed them?

It can take some time for them to show up in the app, it's not immediate, so check back in a day or two and see if they populate.