I've started observing a situation in which the polling doesn't start if the job is currently building.

The job is configured with the "Execute concurrent builds if necessary" option checked, so I would expect that it runs additional builds immediately after a github hook arrives (a push happens while build is running). However, what I'm seeing is this:

If the job is not building and the hook arrives - the build starts.

If the job is building and the hook arrives - the hook is noticed but the polling doesn't happen, and the additional build doesn't start until the current build finishes. After it finishes, the poling starts, the change is found and the new build is started.

If this is the intended behavior, what is the rationale behind it ?