Thanks Dave M for your reply.
I have since looked at various other sheets (including Lighthouse Grande (PET?!) and 100% polypropylene pages) and seen the effect there too. It is probably (hopefully!) nothing of concern particularly if you can also see it on Mylar holders.
I'm still searching for archival quality album pages (not holders) so if anyone out there has recommendations, please post! I would have gone with Lindner for my expensive notes, but reference on their website that pages are made out of "hard PVC" makes me a little wary.
The obvious solution is to use Mylar holders and insert them into the pages, but on the holders I use (Lighthouse 100% polyester) it did seem to cut down the visual appearance, so my preference is just to use one holder, ideally the album page.