Bug in CGPDFDocumentRef?

My iOS App (iPad) is rendering a multipage PDF using CGPDFDocumentRef. In iOS11 and previous it renders the images perfectly but in iOS12 and iOS13 it renders the first page in colour but the rest are monochrome. Now in iOS14, even from the first page, the images are either completely black or are distorted with diagonal black lines running through them. Any blocks of a single colour are missing. (This is an App for schools, educational content, and most schools are hesitant to upgrade their iOS so I have to support all the way back to iOS8.)

The pdf was generated in Adobe InDesign on Mac and I've checked the pdf file in Preview, Adobe Reader and Adobe Acrobat on macOS and in Adobe Reader, Edge, Firefox, Chrome, Opera and Brave on Windows 10 and all is perfect.

Any help appreciated,
Thanks - Paul