It seems that since Android 12 there is a problem to import routes exported from a different application, which has been downloaded or sent via email and stored on the phone's storage.
The application can import files exported from Fly Is Fun but cannot see files saved by other applications. It would be nice if there would be a folder with special permissions which allows FIF import files, or the possibility to import the route from the clipboard - so the file could be opened in the text editor and a route copied into the FIF skipping file permissions.
I checked this issue on my previous phone (Older Android) and it's working correctly there - I can import files sent via email.