Resolution: Not A Defect
When I add the branch property "Suppress automatic SCM triggering" (which was introduced with
JENKINS-32396) to a multi-branch pipeline job, then it does not react on push notifications at all anymore.
It does suppress the trigger of a build after branch indexing, as expected. But if I setup a jenkins hook e.g. in stash/Bitbucket it does also swallow all my pushes. If I push a new branch or a change to an existing branch no build is triggered.
I would expect in this case a build starts, and only if a branch is detected during indexing t get suppressed.
If I'm not mistaken the code should be here (and looks actually good to me):