We could reproduce this issue at our end, following are the findings
1. This issue is reproducible in CPU, GPU Rendering and Print Preview(thumbnail).
2. Reproducible in older versions as well including Illustrator CS6 (not a new break in CC 2018)
3. Works fine in Overprint Preview
4. Works fine in Print & PDF
Please confirm above observation match with the problem reported in this issueShawn Anderson supported this idea ·
An error occurred while saving the commentShawn Anderson commented
This issue is still there in CC 2020 and and does affect PDF export for output. It looks like it also happens when you combine overprint on a fill and multiply.
When a spot color is put on top of an image and set to overprint or multiply over a CMYK image, the top spot color element should have no effect on the item underneath. If you stick to a single setting (one multiply or one overprint) it's fine but if you combine both or put two multiply effects in place the background image prints much darker.
See attached file. The top three examples are incorrect the bottom two are how it should work in all situations.