Seem to have found a bug; or does anyone know the possible cause.
I created an entry in the placement.info:
<Place Fields_MediaPicker-HeaderImage="/Header:before" />
(no other entries)
If I remove this - then the Zone header is properly rendered/shaped (because I see the <div class="zone zone-header> being created; but when I use the above placement - I no longer see this; which then prevents my Zone-header.cshtml from being