At least, I think that's what these are. I don't recall ever seeing this before.
Two images attached, cropped from a recent rendering: an original and one that's been darkened to show the edges more clearly. The lines seem to occur only in the clouds (Easy Cloud).
I just noticed this, so no more info yet.
This isn't a big problem, because it's a small part of the image and the lines are virtually invisible. If they do show up they can easily be retouched out. But has anyone ever run into this, and is there a fix?