Are you using a VPN or other proxy service of some sort? Do you have your browser set to sync with the same kind of browser on other device(s)? Have you checked and fiddled with your iOS devices settings for the browser app you’re using while having this problem to see if adjusting the browser’s settings and restarting your device solves the problem? Have you cleared out any stored passwords for FT that may reside on your iOS device and/or on the iCloud?

Maybe try to use a newly installed browser of a different sort? Say using Firefox if you’ve been having this issue while using Safari for FT, or using Chrome if you’ve been using Safari (or Firefox) while having the problem.

An iOS update and/or app update has at times meant that I have to log back in (and repeatedly at that at times) to a site, but more often than not, this hasn’t been the reason that I have had to log in repeatedly. VPN/proxy use and browser sync settings have more frequently been the issue for me before when it comes to FT use. That and some kind of other weird stuff. But it’s been a long time since I’ve had this kind of thing knock me out as logged in.