bug#50907: 28.0.50; mpc: Can't add album to playlist

[Stefan Kangas]

I can't add any album to the playlist in MPC.

    0. emacs -Q
    1. M-x mpc RET
    2. move point to "Albums | Playlists" window
    3. Pick any album, press RET

The result is that the playlist clears in Emacs and the songs get added
into the buffer.  However, if I try playing any song in the
"*MPC-Songs*" buffer, I get "This song is not in the playlist".  The
songs are also not added to the MPD playlist.

I expect that the playlist a) does *not* clear (items get appended
instead), b) that the songs are in fact added to the MPD playlist and
c) that I can play the songs.

---

Maybe related:

If instead of pressing RET in step 3, I type `M-x mpc-playlist-add', I
get a message like "Appended 12 songs", but the old playlist is
unchanged in both Emacs and in MPD.

I'm not sure if my MPD client is in a weird state or something, but
ncmpcpp is still working as always.  What can I do to investigate this?