A quick Google search will relay that this is quite the common problem for listeners across virtually all streaming platforms, regardless of choice, including Spotify and Apple Music, Tidal and all the rest. Hey the artist may have taken the music off the platform for unknown reasons. Favorite tracks might be missing due to discrepancies with contracts and distribution deals. All most likely associated with licensing. Annoying yes, but welcome to streaming. I do like my backup of CD's and LP's and downloads just in case.