I'm running Subsonic 4.7.beta2, and I noticed this issue today. Here's my procedure to replicate:
1. Create a new playlist. Do NOT mark it as public.
2. Queue up a song not from the playlist
3. From the More Actions dropdown, select Add To Playlist
4. Select the playlist you just created.
5. The song will NOT appear on the playlist.
I didn't see anything in the changelog relating to this in beta3, and I didn't see anyone else mention it on the forums, so I thought I'd see if anyone else was seeing this.
