Previews - Spot colors cause undesirable results

Why parts of your layout are not visible in preview of a layout, PDF or EPS

Details see https://helpx.adobe.com/indesign/using/spot-process-colors.html

"If an object contains spot colors..."

Hint
EPS and PDF have the same basic structure, so this means on PDF you also may have undesirable results "when using an application other than Illustrator or InDesign".

This is the reason why previews fail here.

Converting spot colors to process colors should solve this issue.