It’s a weird quirk resulting from Roon’s flawed Tidal integration. There appears to be two types of references toTidal songs - one that’s part of your library and one that’s not. Both of these reference types can be added independently to a Roon playlist - even if they both point to the exact same Tidal song. A library reference will track play counts and heart tags. The non library reference will not.
If a Tidal song (not on your library) is playing on Roon radio, then “add to playlist” adds the non library reference. OK - once you know this, you might be tempted to “first” add the song to your library, and “then” add it to a playlist, however that is also wrong, because Roon radio is still playing the non library reference which is what will get added to the playlist. So you have to “first” add the song to the library, then “search” for the song, then find it in your library, and then add the library reference to a playlist!
All this could be avoided if Roon simply said “Add to Playlist” implies “Add to library” and performed both functions. This would simplify the user interface and also make this consistent with only library references in playlists with consistent functionality (play counts, hearts etc)