My experience with both PL100 and HP5+ is that skies give problems with this method of development. I have also noticed the same artifacts on a couple of shots containing a large amount of smooth beach sand, but that problem is not consistent, so I may have screwed something up. I have not noticed any problem with large areas of ground vegetation (the prairies we have in Florida). For negatives with a large amount of sky, I have returned to brush development.
juan