A similar situation is here: https://www.openstreetmap.org/way/491657992 or http://map.openseamap.org/?zoom=17&lat=51.39211&lon=3.42444&layers=BFTFFFFFFTF0TFFFFFFFFF
Just the parts above the groynes are pitch-green. In the past (a month ago, or so) the whole area was green.
This is/was for kite areas at sea, the kitesurfing areas in inland waters were/are not rendered at all.
So I assume this fails due the render rules.