Somewhat confusingly, the service road https://www.openstreetmap.org/way/733298699 was part of the Clwydian Hills relation https://www.openstreetmap.org/relation/10757713 and extending it meant that that boundary was no longer a valid polygon.  I've cut it in too and removed the end part from the relation in https://www.openstreetmap.org/changeset/166101884; the boundary should reappar now.