I've done some testing and it's a scanning issue and, thankfully, one which can be fixed. The image above was scanned on my Coolscan 9000 with the Image Mechanics wet mount tray without any masking around (which I normally don't do).
I have now tested with the 869S and Focal Point glass, which evidently doesn't work so well regarding Newton rings. The discolouration is still present.
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Then I tested with the 869GR tray (which deals with Newton rings much better but is a hassle to use with film strips). This tray has a special square mask that one puts on top of the negative. This did a better job to remove the discolouration but there's still some.
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Finally I scanned with the Image Mechanics tray again but now with the square mask on top. The discolouration is still present BUT look at the top, it's only in the middle. This is because the edge of the square mask isn't straight, rather there's a part cut out in the middle. That's what's showing only in the middle along the top edge of the frame.
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So I need to devise a simple mask for use with the Image Mechanics tray, which is the one I prefer to use because there are no Newton rings and I can scan a lot faster than with the 869GR. It probably needs to be a bit wider to ensure no stray light comes in from the sides. Exciting stuff.
Thanks everyone for your help and for putting me on the right track to fix this.
best
Philip