When "# needsrootforbuild" in OBS does not work...

...always remember, that you also need to change /usr/lib/obs/server/BSConfig.pm:

# Allow to build as root, exceptions per package
# the keys are actually anchored regexes
our $norootexceptions = {
        "my-project/root-package" => 1,
        "dev-projects.*/other-package" => 1,
};

I already forgot that and wondered why it worked for "root-package", but not for "other-package" (which was not yet added...)