OnSongChange / OnStop only work in foreground

Hi,

Version: mocp svn pull earlier today.
zsh 5.6.2
Slackware 14.2 64bit.
screen 4.6.2
TERM=screen.linux (ssh'ing to server from plain tty/framebuffer)

I added some commands for OnSongChange and OnStop:

OnSongChange = "/home/david/scripts/moctitle %a %r %t"
RepeatSongChange = yes
OnStop = '/home/david/scripts/moctitle'

The moctitle script is just a short shell script to print song details to a
file in /tmp for reading by another application.

When it seemed not to be working I tried running mocp -F -D, and discovered it
works perfectly when running in the foreground.

I also ran it with only -D, and searched for OnSongChange in the scrollback,
but there was nothing.

Any ideas, or can I provide more info