Given a Jenkinsfile of the form:

node {
    sh "ls"
}

If I configure a multibranch pipeline pointing to a git repo with this Jenkinsfile, Jenkins does not show changes. I have to explicitly add a checkout step:

node {
    checkout scm
    sh "ls"
}

If Jenkins cannot find changes, it cannot notify the committers that a build has failed. For example, CulpritsRecipientProvider in the email-ext ultimately calls WorkflowRun::getChangeSets. This returns an empty list, so Jenkins doesn't know who to email.

Adding 'checkout scm' seems redundant, because Jenkins has found the Jenkinsfile from git already. In principle, Jenkins has the data it needs to work out the changes, as far as I can see.