Thanks for the report, the main thing here is that you are asking SongKong to identify an album, but it isn’t actually a real album just your own rip of a collection of songs. To be fair you do set Nur dann Entsprechung, wenn alle gruppierten Songs einem Album zugeordnet werden können (Only allow match if all songs in grouping match to one album) to Nein to allow partial album matches, but still working against the SongKong way of doing things. Songs can appear on many albums and if you only have one song there is not much to help identify the song and then even less to help SongKong get the original album.
And that is the main reason that in this report artwork is only found for 60% of the songs.
So notice here that there are three songs that could not be identified to an album at all so these don’t have artwork
One (You’ll never Walk Alone) was matched to an album with no artwork
However the match to The Longest Johns does link to a release with artwork but SongKong does not have it and logs show
01/03/2022 10.47.59:MEZ:MusicBrainzUpdateSong:lookupDirectArtworkForRelease:WARNUNG: Unable to create BufferedImage:http://coverartarchive.org/release/9ff59752-ae14-436d-9cfa-538c3a277a49/front
When I loaded it in browser the images redirected so my guess is SongKong cannot handle the redirect, have raised this issue
So is this what you mean when you say it doesnt sync to music app, or do you mean there are files with artwork in SongKong but not shown in Music app ?