You may change the default branch on your VCS provider, and sometimes that change won't be reflected on CircleCI. This can cause issues with some features that rely on knowing which branch is the default, such as Github status checks and Build Only Pull Requests.
To view the current default branch, you can use the API endpoint under default_branch:
We will check the default branch and update it for your project on each commit pushed. If you change the default branch it will be updated on the next commit push to CircleCI
At the moment, to update the default branch in a Bitbucket project, we need to manually refresh the project's information on your behalf. Please submit a ticket to Support or write to firstname.lastname@example.org.