Just noticed that on iOS you can heart tracks when listening to a playlist using the ARC app, but still strangely not when using the roon app - really hope they can implement this feature on the roon app soon.
Can you be more specific on which Roon app is having the issue. I just hearted several tracks on the Q screen while playing a playlist using a Win PC client.
Remember, you cannot heart a track which is not IN your library, even if it is on a playlist.
Easy to guess, he means directly on the play screen which is only possible on ARC but not on the other Roon Clients. I find it too very annoying to change every time to the queue screen just to heart a track.