/etc/network/if-up.d/mythtv-backend.if-up.d is a bad idea

This file was put in for MythTV0.21 and has never worked (or been tested) The filename is not valid for run-parts (see the man page) and so it is not run. I don't want you to fix the name but remove it altogether. There are much better ways of making mythtv wait for the network to come up (e.g. systemd) and even if they don't work it is a really bad idea to just restart the myth backend process just because a network link has appeared, you could interrupt a recording. Any replacement for this should check that mythtv has come up and is not busy first.