Twitter for iOS was reportedly showing a bizarre bug where users on iOS 15 were logged out from their accounts. Many users reported this issue and Twitter has said the matter is under investigation. There is no detailed information about the bug as of now.

source https://gadgets.ndtv.com/apps/news/twitter-bug-logout-ios-15-iphone-ipad-app-multiple-accounts-support-2624052#rss-gadgets-all