To anyone who finds this old thread, I have *finally*, after much travail, found a real solution! The key was to navigate to the settings in F-Stop and select the Main -> Check for Changed Images -> Compare Image’s Dates option. That alone won’t do it, of course, but then I used the free Android App named Total Commander to manually and recursively change the datestamp on the folder which held all the photos which I wanted F-Stop to scan/rescan. As soon as I did that (a few seconds for 25,000+ images), I restarted F-Stop and voila it rescanned everything successfully.
I really wish the developer would respond to support emails, and better yet add a feature to do exactly this, but in any case there is a solution available (at least for Android, not sure if F-Stop is a thing in iPhone).