It will either be a bug or a misconfiguration.
This is an alpha rule which has not had any significant update since 2015, so it may have broken at some point.
Access control testing is something that we would love to get back to and improve, but right now its not something that I'm likely to be able to find time to look into.
If you do decide to try to debug it then we would be very interested in what you find :)
Cheers,
Simon