Because v6 uses the new version of Android storage access and requires explicit permission, the links set in the old version of the app are not able to be transferred automatically unless you stored them in the OpenSong/Pads/ folder. This is part of the reason I stopped updating v5.6.0. There were so many parts that couldn't easily be changed to meet the newer Android security requirements, particularly keeping permissions for external files. v.5.6.0 always encouraged you to store custom pad files in the OpenSong/Pads/ folder (as does v6) as when you set that storage permission for the OpenSong/ root folder when you first run the app now it keeps permission for all folders and files within this root OpenSong/ folder.
You don't need to set pads individually for songs though, you can use your custom pads and assign them for a specific key and then as long as each song has a key set, it will use the 'auto' pad and that will use the custom file you have set for that key, otherwise it uses the built in auto pad that is bundled with the app.
This is the case, unless you have actual individual link audio file backing tracks for each song. Again, these should have been stored in the OpenSong/Media folder, but if you stored them elsewhere (i.e. not in the OpenSong directory), you will need to give permission again. This would also happen again if you uninstalled/reinstalled and is a restriction from Android to make sure apps don't get permanent access to your files. If you do want this, make sure to use link audio files that are stored somewhere in the OpenSong/ directory structure.
Check out this page of the manual: