Forking external project to private org fails

I’m attempting to fork an external project to our corporate/private “group.”

We have enabled the option to prevent project forking outside of group. However, attempting to fork an external project to our group fails with a generic error, as shown below.

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We are using the hosted version of GitLab (15.x). I’ve tried forking the project to my personal account, and it works. It has existing forks, so it appears to be allowed. I’ve asked the community leaders if forks are permitted, and they replied “welcomed and encouraged.”

It seems to be an element of “prevent project forking outside of group” that is not clearly delineated in the documentation of the feature. Can anyone elaborate?