Hi! If I can add my experience on this problem.
I use Spotify both on my PC Windows and on Smartphone Android. I have some music that is released only on cd so I've added it to a folder on my PC that Spotify will read and I have it on my "local folder" playlist inside the app.
So, I normally use the automatic playlist "liked songs" (I think that is this the translation) and I'm used to like also the local folder songs, since in this way, Spotify automatically add the song to that playlist.
Then I simply put the songs to a folder on my SD for the Smartphone and the magic happens letting the Android app recognize the files and play it without any other things to do by me.
Some of theese local songs are also in another playlist synced local to play it offline.
Then I've repeated the process recently for a new cd (late genuary) and the magic no more works. On PC is all ok, but on smartphone no, the new music aren't recognized (i can see them in the local folder and reproduce them but on the "liked" playlist the songs aren't available). Doing some test, I've noticed that putting a like on the same local song both on PC and Smartphone, duplicates the row in the automatic playlist, and the song from the other platform (respectively) is not playable. This behaviour afflicts also the "offline" playlist the same way of the automatic.
To fix this situation I've created a new playlist "liked song smartphone" where I've putted most of all the liked song from the automatic playlist (via PC) and then manually added the songs liking them from smartphone to that specific new playlist.
I think that there is a problem on the pattern matching that the app uses to recognize the name of a local file or in the path that it uses to save the song, so basically the path from pc is different from the path on smartphone that leads to two different songs even if they are basically the same (or why not both the problems).
Added some sample screenshot to better understand the situation. I hope that this can help finding a solution or fixing the problem through code.