The redirects don't work at all, and the company shows no signs of finally fixing this problem.
As a workaround, I linked the .htaccess file in the root directory as a symlink to the subdomain directory (in /subdomain.example.com: ln -s ../.htaccess) – at least this way the redirects work.
Whether this also works with alias domains needs to be tested.
If there are many subdomains that all need to be redirected, it may be better at the moment to create a directory, e.g., “redirects,” symlink the .htaccess there, and assign all subdomains to this directory as the root directory.