I don’t understand how updating to the paid app would have affected your tags at all. The free app just looks for the key to unlock features and remove ads. Did you uninstall the free version first?
By default, F-Stop saves all the tags and ratings to its internal database only. This is how the free version works as well. When a file has changed tag and rating information, we overlay an “Update Metadata” icon on the image.
Please see the following page for more details on how we handle metadata:
From the options you mentioned, you do not have the option “Autosave” checked which would require you to save them to disk. This is accomplished by going to the “Update metadata” link from the navigation drawer, selecting the files to to update and selecting “Save metadata” from the menu.
I am not sure why full scanning was triggered for you by just updating one file. If this is repeatable, would you be willing to install a test version of F-Stop so we can log what is happening?