Outside collaborators see an error when clicking on the invitation link, but the repository itself is created and available via link

Everything worked fine until the new year, but recently, when outside collaborators clicked the invite link, they started seeing an access error screen ("Repository access issue. You no longer have access to your assignment repository. Contact your teacher for support"). However, their fork is created in the classroom, and they can access fork via the link and make commits. All their repositories are private.

Start by reproducing the outside-collaborator invitation flow using a private assignment repository, then compare the invitation-link result with the repository fork link that works. Trace the GitHub Classroom CLI path responsible for invitation and repository access; done means outside collaborators can follow the invite link without the access error and can reach their created fork.

Written by the indexing model from the issue text.