Whenever I use mpc next or any other player (ncmpcpp) MPD will play the next song and will remove the previous song from the playlist (generated by mpc add /).
My mpd.conf is located at /etc/mpd.conf
And I've not found any directives associated to this annoying behavior. I'd prefer that MPD jump to the next song but without removing any previous songs.
Any help is welcome.
After reviewing yet another time my settings, I discovered ncmpcpp was in consume mode (it's toggle by pressing R). I hope this answer helps someone else.