Apple Music / iTunes: Cannot add track to playlist when iPhone connected via wifi
This problem has been bugging me so much, it seemed so random but yes it definitely disappears the moment you plug in the iPhone, then appears again the moment you take it out.

I assume this is a bug and will be fixed soon, but here are a couple workarounds:


As you say, eject the iPhone either by physically disconnecting, or clicking the eject icon.
You can click "Add to My Music", then go to your library, find the relevant song, and drag/drop into a playlist. It works, but it's not much fun.