Basically, the issue is well described at JENKINS-39925 which is close as a duplicate of JENKINS-40653

The thing is that JENKINS-40653 assumes that currentBuild.result variable is set to SUCCESS. If your pipeline ends succesfully but you didn't set the currentBuild.result value then the email won't be sent to the users who committed since the last non-broken.

The problem seems to be in this else if. Switching the else if to an else seems to fix the problem.

Probably this is failing because the plugin was designed originally to work with Freestyle jobs and the email was sent as a post-build action, in that case the build result s always != null. That's not works at Pipeline.

I will try to send a PR shorly for your review.