Changeset not reflective of Pull Request

I'm using Blue Ocean's default multi-branch setup for GitHub. I am using Jenkins to build and test Pull Requests from GitHub.

I have a repository with multiple subproject directories that I would like to selectively build and test depending on changes in the Pull Request. Looking at the documentation it seems like I should use `when { changeset 'my-dir/**' }` to do this; however the changeset does not contain the changes from the Pull Request, but instead contains the changes since last build + the changes merged to master since the last build.

I think the behavior here should be to consider the changes in the Pull Request to make it possible to use this conditional.