Pagure still shows master when you go into a branch for a PR, as per the image, and when you click "Create pull request" you get a "Bad Request: Branch master could not be found in the target repo" error.
It seems it doesn't lookup what is the default branch.
This is a duplicate of https://pagure.io/pagure/issue/5120 actually.
So I'm going to close it as such.
Thanks for the report though!
Metadata Update from @pingou: - Issue close_status updated to: Duplicate - Issue status updated to: Closed (was: Open)