i wanted to echo the actual solution posted earlier in the thread! A simple phone reset solved this for me as well, yay for forums!
so for clarity and to be specific: I would be listening to a apple curated playlist from for you tab, click dots and add to playlist to add a track I heard (that was not in my library) to a playlist I made (like to listen), and it would not work. The work around if adding the track first to my library was multi stepped and annoying.
WIth reset, the same steps above successfully add the song to your playlist even if not in library. NOTE: was instresred to find this action ALSO adds to your library, nice that one action does both, bummer if you were hoping to add a track to a playlist without adding to library.
RElated possibly helpful info: adding a curated playlist to my music seems to add to your playlists, but not to library. NICE! I prefer that. How're, as soon as you mark that playlist for download to device, all tracks are added to your library. Now if you choose to remove the playlist, you must also remove each individual song from your library. NOT NICE. Wish playlists were separate like in beats.