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}
--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)