In https://jira.atlassian.com/browse/BSERV-9728 support was added so that, when the default branch of a mirrored repository is modified, the mirror node can receive the notification and update its own default branch for the repository.
However if the default branch was modified prior to mirroring the repository, the mirror node does not pick up the change to the default branch.
In this case a workaround is to update the default branch again after mirroring the repository. (For example, it is sufficient to navigate to the Repository Settings page as a user with admin permissions, and just click the Save button without even updating any value. The notification to update the default branch should then be sent to the mirror).