My guess looking at the area is that the photography is a mix of different dates. In the vicinity there are cloud shadows visible from clouds that are not visible and some small sections of strong cloud. See east and north of this area for example.

So I think it is simply that this area was left uncovered by cloud free imagery so there was not alternate. You couldn't miss it in quality control it is too obvious. They would also have eliminated the cloud shadow images before producing the ortho if they had the chance.

The edge of the area has the blend effect due to feathering and seam smoothing with the surrounding images. This is also why the edges of the cloud image are not straight.