In the console for branch, there is no mention of start up of configuration builds, only that they are completed.

As a consequence, you get the following logs :

Cleaning workspace
Resetting working tree

then it is hanging for the duration of default #10, then displays

Completed default #10 Result SUCCESS

I would add something like

Waiting for the completion of default

just before hanging, similarly to what is logged by the parametrized trigger plugin when a build is blocking on the completion of another.