I configured Jenkins to trigger on changes to GitLab on push and opened MR events.

I can see that gitlab itself correctly invokes the "merge request hook" of our Jenkins instance. However, when I attempt to open a merge request into the master branch, even though the webhook is send (HTTP 200), Jenkins does not start the respective build.

If I edit the base branch of a given MR from master to develop, gitlab re-issues the webhook, and Jenkins starts the build as expected. Is there some kind of setting I need to tweak in order to allow MR into master to trigger build jobs, or is this some kind of bug?

Use case:

Using git flow, we currently open release/X.Y.Z branches for new versions. In order to test them, I want to run the full test suite / Jenkins pipeline for this merge request until all tests are passed and the merge request can be accepted into master.