Export PDF result is not page size

When I use File/Export the resulting PDF shown in Acrobat appears to have the InD page size.

BUT when the PDF is imported into InDesign, the page boundaries are incorrect.

I have to add a white background to the page in order for the PDF to capture the page size. 

I've used InDesign for, well, decades and this is unusual.

Indesign file view:

View in Acrobat - correct margins 

Export PDF imported into InDesign: page margins wrong

Examples PDFs: top with white background added to page; bottom without background