they actually do work, but only under powershell. There's also a tar.exe but I haven't managed to get a valid tarball out of it.
How are the tarballs you get out of it invalid? Do you have a sample file I could take a look at?
FWIW, since it is bsdtar it supports a few additional options[1] to control the format. You can pass --format=gnutar, --format=zip, --format=ustar, --format=7zip, ... for a bit more flexibility and to create archives in other formats.
[1]: https://man.freebsd.org/cgi/man.cgi?query=bsdtar&sektion=1&f...
I'll see if my old devops code history can produce a bad-state tarball and get back to you.