On systems whose sources are stored in deb822 format, apt-add-repository
commands that act on existing sources (such as `apt-add-repository
--component contrib`) fail silently: They don't update the entries, but
don't report an error either.

I didn't go though a full fresh install to show its reproducibility, but
it can be shown easily using Docker images:

$ podman run --rm -it --network=host docker.io/debian:sid
# apt-get update && apt-get -y install --no-install-recommends software-properties-common
# cp -a /etc/apt/sources.list.d/ /tmp/
# apt-add-repository  --component contrib
[Enter]
# echo $?
0
# diff -r /tmp/sources.list.d/ /etc/apt/sources.list.d/
[no differences]

This is probably not trivial to fix given that software-properties deals
in more than just this single tool. As a stop-gap measure against silent
failure, operations on all repositories could start to fail once any
`*.sources` file is present.