Summary
As a bitbucket-branch-source bug:
For a multibranch project stored in a Bitbucket repo, the URLs in the <actions> sections of jobs/the-bitbucket-multibranch-job/branches/master/config.xml are incorrect. This incorrect URL takes the user to a PR builder page on their Bitbucket server, instead of just taking them to the page for that branch.

Discovered while running bitbucket-branch-source 2.2.9, but it looks to be present in master too.

Frequency
100%

Steps to recreate: these are still a valid way of recreating the issue, just keep in mind this no longer looks like a Blue Ocean specific bug to me.
1. Connect to a Bitbucket repo with a functioning Jenkinsfile via Blue Ocean, and run that pipeline if you haven't already.

2. Click a successful run of master (or any branch really, it doesn't matter which) to produce the details view, which looks like this:

3. Click the button next to the branch name, up near the top left corner of the page. The tooltip says "Opens branch in a new window:"

4. That new window will take you to what I call the pull request builder on your Bitbucket server. The URL that button is pointing to is https://bitbucket.beescloud.com/projects/FEB/repos/jenkins-49409/compare/commits?sourceBranch=refs%2Fheads%2Fmaster. It'll be pre-populated with settings to create a master to master PR, which doesn't make a lot of sense:

5. This issue only manifests with Bitbucket repos. With GitHub Enterprise, for example, you'll be taken to https://your.github.server/org-or-user/another-blank-repo/tree/master. A sample URL might be https://github.yourcompany.com/dev1/another-blank-repo/tree/master: