I think I can confirm it's areas cast in shadow, because this version rendered in only 1hr 15m in SR vs about 2 and a half hours. Additionally, the area in light where the monk/nun is, rendered and finished (to the bottom of the render frame) before the area just to the left (which had 3 rows left), with, of course, some shadow. This all being translated to PT I could see it being why, and my Dune scene was similar circumstances where because of the sand and low sun, the ground was mainly cast in shadow. A shame really as I liked the idea of looking out into the valley and mountains in a dreamy light.
Interestingly this doesn't seem to effect shadows at a distance? They all seem to render fine. Just up close, so maybe something with the adaptive sample/geometry.