Steps to reproduce

1. Create a Multibranch Pipeline that has at least one open pull request (you can do this easily by creating a new private repository, editing a file and submitting the change as a PR)
2. browse to the pull request tab in classic for the pipeline (URL should be something like /job/i386/job/app-store-demo/view/change-requests/)
3. Click "Open Blue Ocean" and be redirected to the branches tab

What you should see
Open Blue Ocean redirects to the Pull Requests tab (e.g. /blue/organizations/jenkins/i386%2Fapp-store-demo/pr)

Notes
Originally was a Bitbucket issue but also applies more generally to any branch source plugin, including Github.