title: Jenkins multibranch pipeline jobs not detects changes
body: <p>Dear Jenkins maintainers,</p>
  
  <p>I have a CI system where hosted BitBucket server able to notify the Multibranch Pipeline about the Pull Request changes.</p>
  
  <p>When I click "Scan Multibranch Pipeline Now" everything works as expected. The scan searches for BitBucket PRs and only finds the ones which contain the Jenkinsfile.
  Those ones trigger a job immediately.</p>
  
  <pre><code>Checking PR-10 from PROJECT/repo and branch my_branch
  Changes detected: PR-10 ({old_hash} → {new_hash})
  Scheduled build for branch: PR-10
  </code></pre>
  
  <p>However, If Jenkins was notified by webhook, the job is not triggered.
  I see in the "Multibranch Pipeline Events" that the notification has arrived, and it is checking the existing PRs. Actually, the checks find no changes on branches, which happens with "Scan Multibranch Pipeline Now".</p>
  
  <p>Do I miss something here?
  Why the notification event does not behave the same as the scan and triggers job corresponding to the changed branch (PR)?</p>
  
  <p>I appreciate any answer and I can share deeper details on request.</p>