I've spent a decent amount of time researching this issue in the Spotify community and elsewhere and I've tried all of the solutions suggested to no avail.

I have Spotify Premium.  I download songs to my SD card which has 157+ GB of available storage.  I download playlists to my SD card with "High quality" and over Wifi and it stays for the first day or two, but when I go back into Spotify after that they are gone and I have to re-download.  I have A LOT of tracks on my playlists and having to download them again and again is VERY annoying.

I've tried:

1: Clearing cached data, memory, storage whatever you want to call it.

2. Uninstalling and re-installing Spotify

3. Formatting my SD card (which required me to move ALL of my photos and videos to another memory card which was NOT fun)

4. Resetting ALL of my offline devices

5. I confirmed that I'm not downloading more than 3,333 songs (which seems to be the offline track limit)

I don't know what else I can do at this point but as a paying customer, this is EXTREMELY annoying and the fact that the same exact problem has persisted for over a year now with no known solution is very unsettling.  Spotify needs to get this issue fixed once and for all.  The ability to download songs and listen to them offline without sacrificing data usage is one of the main reasons why people pay for the premium service and this is not acceptable.

I have the Galaxy S8+ Android Version 7.0.