Lines along sides of frames

Most of my rolls (I haven’t confirmed if all rolls have this problem) from various cameras and stocks of film, have light whiteish lines along the sides. Originally I thought this was 100% an NPL issue but after very carefully inspecting my negatives, I realize that NPL make the issue more visible after conversion but it is not caused by NPL. It was hard to see the lines in the negatives due to the inherent low contrast of negatives. The lines are most visible in the sky of photos. I have attached an image where the lines can easily be seen on 3 of the 4 sides. I have attached another image and the negative where the line is very visible on one side. I use a Digital Camera (Nikon D810) to scan my negatives and can see the lines on the original (unconverted) images of the negatives and on the negatives themselves. I also notice the lines even on half frames that I have cropped (2 images in the original digital image from the camera) so I believe it is not an artifact from the digital camera. I can’t ever see it clearly on 4 sides but I can see it clearly on 3 sides of some images (such as attached). It is perplexing the this occurs from various cameras and stocks of film. This makes me think that it is a problem related to the cameras themselves? But how can this happen with various cameras that are so different (for example a Nikon F80 and a Konica Recorder)? Any ideas what might be causing this? Any way to fix this besides by using the content aware tool in Lightroom that is quite time consuming given I see this on most images?
Note: I am a new user so I was only able to add one image in this post

Here is another image

Here is the negative of the last image

Welcome ro the forum @magicspacerobot

Do you still get these lines when you don’t use a film holder?

Hi @Digitizer and thank you for the fast reply. I am sorry it took me 3 weeks to get back to you. That is a very good question. I am going to try to determine a way to test this and will update you with the results but it may take some time.