If you have followed the "delete index.dat" solution:
to fix problems with local files not appearing in Spotify, but are left with strange bugged unrecognizable characters in "title", "album" and "artist", you need to:
1. Remove the folder from Preferences in Spotify, close Spotify application
2. Rename file name, and right click the Windows 10 song file -> Properties -> Details and edit "Title", "album" and "Contributing Artists" to anything else. (I used mp3tag to change info when some files somehow got locked for editing)
3. Start Spotify, add the folder again. Check that the songs that had broken info are now showing what you set it to.
3. Remove the folder from Preference again, close Spotify application again...
4. This time, edit the file name, "Title", "Album" and "Contributing Artists" in desktop to the correct information
5. Open Spotify again and add the folder to Preferences. This time the songs info should be what you set it to.
The reason you have to first change the desktop files info to something that is wrong, add it to Spotify, remove the folder and then make it right and add the folder back agian - is because you must make Spotify think the files are supposed to have another name and information. If not, the songs will just not show up again, or keep having the corrupted names.