Please find attached a few contact sheets along with a few images from the contact sheets that sort of tell the story of why I came to the conclusion that both yourself and I have suffered from the same issues with the Pentax 645n2 fitted with the 45mm lens with Petal Lens Hood. It is worth noting that these negatives were made in 2003, and I was very new to photography and the darkroom, and also the Pentax was a new toy to me. These negatives were only the 15th roll on-wards of film that had gone through the camera. I was very confused when I developed my film as to what could have been the problem, so will explain how I came to the conclusion that the issue was the Petal Lens Hood and nothing to do with my development routine.
If you look at the first two contact sheets you will notice that not all of the images suffer from the issue, that is because the images that don't were made with the 150mm lens, and all of the images with the issue were made with the 45mm lens complete with Petal Hood, which points to an issue with that particular lens.
On the third contact sheet shown, all of the images were made with the 45mm lens with Petal Hood, but while exposing this roll of film I noticed that the lens hood was fitted incorrectly, so put this right, which as I have mentioned before, is done by lining the white dot on the lens with the white dot on the hood and turning clockwise so that the word "Pentax" sits on top of the hood. You will see that by doing so the next two images made, which are the two on the top of the right hand column of the contact sheet, are without issue.
From studying these contact sheets I sort of came to the conclusion that by not having the Petal lens hood fitted correctly, along with a harsh side light, these issues would arise, though if you look at the river image that I have attached, scanned from one of the contact sheets, that is not strictly true, as there was no side light on this particular occasion.
Since making these images, when using the 45mm lens with Petal Hood I always double check that the hood is fitted correctly, and if there is a harsh side light I use my hand to cast a shadow over the glass, and I have never had the issue since, some nine years later, so must be doing something right!
I have also attached some images made from those contact sheets to prove that a silk purse can be made from a sows ear (IMO)
Pentax_645_hood_issue001.jpg Pentax_645_hood_issue003.jpg Pentax_645_hood_issue004.jpg Pentax_645_hood_issue002.jpg Winters-Morning_Elterwater.jpg Elterwater_Still.jpg
I hope this helps.