All the symptoms are the same as in https://issues.jenkins-ci.org/browse/JENKINS-40652.

I have a repository without a `Jenkinsfile`, there is an user with write access to it that creates a PR from a branch in the same repo and jenkins doesn't pick it up, saying:

  Getting remote pull request #6379...

    Checking pull request #6379
    (not from a trusted source)
    Job name: PR-6379
      ‘Jenkinsfile’ not found
    Does not meet criteria

  1 pull requests were processed

If the user created the PR from a branch in his fork, then it works perfectly.