Peter, I tried your workaround. Saving as same name overrides existing playlist. Save as new name creates a new playlist but I still cannot add individual songs to that playlist. Duplicating playlists (two residing in Tidal itself) is not a great working around.
Beyond not being able to pull up resident Tidal playlists directly from the Lumin app, adding songs to specific Tidal playlists seems to be a missing feature. When I hold down on track, I am given several options such as Remove Track (from Tidal playlist) and Add To Favorite (Tidal Favorite) but there is no option to Add To any of the Tidal Playlist. When finding and listening to individual songs, it is a very common desire to add a song to a specific Tidal playlist. Maybe I am missing something. The Bluesound app has Add to (Tidal) Playlist and I used it every time I was using Bluesound...find a great song, choose add to playlist, select a Tidal playlist and click add.
Thanks.