Your mumble.install has usr/lib/*
which also catches usr/lib/systemd/system/mumble-server.service usr/lib/sysusers.d/mumble-server.conf usr/lib/tmpfiles.d/mumble-server.conf as you can see on https://packages.debian.org/sid/amd64/mumble/filelist (at the time of writing this bug report) Please make the mumble.install more selective, e.g. by using usr/lib/${DEB_HOST_MULTIARCH}
