I'm not sure how the changeset is created by the git plugin. I see in https://plugins.jenkins.io/git/#changelog-extensions that I can ask it to compare against a specific branch. But this raises the question of what happens when I have

* master
|
| * my_branch
|/
* old_master
|

and I compare my_branch with master. I don't know the answer.

It seems to me the git plugin could benefit from using the "Reference build" from the https://plugins.jenkins.io/forensics-api/.

Additionally, when creating a multibranch pipeline with a GitHub branch source I can't really see the "Calculate changelog against a specific branch" option anywhere, I don't think it's available.

The current behaviour makes, for example, the first commit of a new branch lack any changeset at all.