Same problem for me...
Instagram suddenly stoped working in full screen. Trying already uninstal/install, instaling previous version of Instagram, but no succes. Instragram is also the only app where I cannot change full screen option.
I had this problem when I just got my phone. I have a Samsung Galaxy A7 from 2019, and there the width of the screen is not similar to other mobile phones. Instagram wasn't, especially in stories. This issue was fixed with the next update of Instagram, so maybe if you wait a little bit, an update will fix the problem. Until that moment, you can disable your Instagram account for not getting notifications from the app until you can use it normally. Or maybe you can find an APK of the app where this bug is already fixed.
Encountered the same issues... Instagram not working on full screen and It seems that only instagram itself can fix this problem. I recently got my iPhone 13 and it had the same problem. This problem also appeared on my old Samsung phone. I tried all those methods that people said but didn't work until I got instagram updated and it finally work! So that's exactly the problems of instagram itself.