POSSIBLE BUG

This was originally brought to my attention by my print vendor, who asked that I correct the overprint and resubmit the file for offset print. As I preflight all my files, I was caught off guard. But on re-inspection I could see the overprint issue.

During preflight process of the InDesign PDF file, a pattern graphic is highlighted as overprinting when selecting Output Preview: Colour Warning: Show Overprinting.

This graphic appears normal in separation view with Simulate Overprinting turned on – the separations and values appear correct. Output Preview: Object Inspector shows the object is false for any overprints (filled path: Constant and filled path: pattern), so this also appears correct.

Examining the original graphic, a pattern created in Illustrator CC 2020, there are no overprints in the attributes panel for the pattern itself (fill or stroke) nor the content box the pattern is filling. InDesign also shows there are no overprints.

I decided to test the overprint by placing an object behind the graphic in InDesign and exporting the PDF again. Output Preview with Simulate Overprint turned on does NOT show the graphic overprinting the object – all separations and values prove this – BUT the Colour Warnings still show the graphic overprinting.

I have opened other files which also contain similar graphics with Adobe Illustrator patterns, and they too show these graphics as overprinting in the Colour Warnings panel, but these files have proven to print correctly with the offset print vendor, ie they do not overprint.

I believe this is a bug that effects containers filled with Illustrator patterns. I have not tested to see if this is only InDesign PDF files or native Illustrator PDFs. PDFs have been created using the Coated Fogra39 profile, and a saved preset I have been using for several years without issue.

Adobe CC 2020, Apple Mac Catalina 10.15.4
Acrobat DC Build 20.6.20042.371807