Applies to: Quark 2019 and 2018
This happens when a document is exported as PDF with PDFX-1a Style standard. This standard flattens transparency in files. The files that were turning black were grayscale tiffs with the spot color assigned to them, and set to have the picture background color as “none”. When Quark flattened the images stacked on top of each other, it was losing the spot color and adding some outline artifacts.
By switching to a PDFX-4 style, the transparency is exported Natively. You can also verify the same under
PDF settings->Transparency, where the transparency gets changed automatically when the PDF Style is changed.