After playing around with it a bit it really appears that after every new "build" or image update you need to re-run (cat <<EOF ipappend 3 EOF ) | sudo tee -a /var/lib/tftpboot/ltsp/{arch}/pxelinux.cfg/default on every build in your LTSP.
After playing around with it a bit it really appears that after every new "build" or image update you need to re-run tftpboot/ ltsp/{arch} /pxelinux. cfg/default
(cat <<EOF
ipappend 3
EOF
) | sudo tee -a /var/lib/
on every build in your LTSP.