iOS 14 introduces some new privacy features and restrictions, which could be related to this issue. You might want to check your iOS/browser settings and see if it is blocking or removing Discourse cookies when you close the browser app.
I’m thinking this could be it, as I see it happening with other discourse forums.
Edit: I have cookies on my phone enabled, and they arent being blocked. Ive also tried a few times by disabling prevent tracking temporarily without any success. This has also started happening just yesterday to me and I haven’t adjusted any settings before hand.
I have the same exact iPhone as you, make sure you are not in private mode on safari, if you are, every time you reload, it will log you out. I’ve been having the same exact problem recently, and it was because of a silly mistake.