Last month I got a Windows update ,after that I can not full screen screen sharing in Teams, the reason is Microsoft enabled new meeting experience, which somehow disabled the full screen mode for screen sharing , I think Microsoft will fix this issue later ,let see.
Anyway ,if you need this feature you can disable that “new meeting experience” ,then you can get the full screen for screen sharing again.
1.Go to Microsoft Teams settings ( right click your Teams icon ,then click settings)
2.Untick “Turn on new meeting experience” ,then restart your Microsfot Teams
3.Done ,check again ,now you should be able to get full screen for screen sharing