Hello, just a message to tell that this issue has been fixed since i've contacted Xbox Game Stack team
Now games made with Fusion works perfectly on Xbox Series consoles.
My two games on the xbox series & one, a platform called "Gofretor" and "Snake Retro", work very good even at xbox one. There's no stuttering but in order to have a working game you need to reduce game objects a lot and of course optimise your events. Unfortunately, Fusion UWP exporter is very basic and cannot compare with other modern engines. But it's quite good for simple games.
Notice also that if anyone wishes to develop for UWP you still can! You have to download the vs2017 and follow Fusion UWP's instructions. I've updated my games and still working on a new update these days.