We started to randomly run into the following issue. Bitbucket webhook sends an update to Jenkins multibranch pipeline, but pipeline doesn't schedule a build because it detects no changes, even though the commit that triggered UPDATE is in bitbucket.

[Thu Sep 10 13:21:01 EDT 2020] Received jenkins.plugins.git.GitSCMSource$ListenerImpl$1 UPDATED event from x.x.x.x ⇒ http://host:port/git/notifyCommit with timestamp Thu Sep 10 13:21:01 EDT 2020
...

git ls-remote -h ssh://user@bitbucket/a/repo.git # timeout=10
...
Checking branches...
Checking branch release/1.247.0
‘Jenkinsfile’ found
Met criteria
No changes detected: release/1.247.0 (still at cc0f29d85ec79db0ef7dc798c8a4e9e2fe0ee410)

There is another commit that exists after [ cc0f29d85ec79db0ef7dc798c8a4e9e2fe0ee410 ], and i can see it with git ls-remote. If I do "Scan Multibranch Pipeline Now" it will pick up the change.
Sounds like a timing issue.