Certain placed PDFs look fine but become strange when part of InDesign export to PDF
InDesign version 18.3. The issue is explained thoroughly in this forum thread, with lots of example exports: https://community.adobe.com/t5/indesign-discussions/pdf-export-messes-up-placed-pdf-and-is-affected-by-mark-bleed-settings/m-p/13957529
Summary: If you place either of the attached PDFs (which were made in Word for Mac), and then try to make a PDF from InDesign, the black rectangle that is in the background of the content will change size unexpectedly. It will always look fine in InDesign but mess up when exported. And the degree to which it changes will vary depending on various factors like marks and bleed, which should have nothing to do with the placed PDF.
Uwe Laubender commented
I can only add that I can see this kind of bug already in InDesign CS6 version 8.1.0 on Windows 10. The effects you'll see when exporting differs a bit depending on the export preset. And of the version of InDesign. For example I could do a good export with a PDF/X4 setting in CS6, but had trouble in later versions of InDesign.
A PDF/X-1a preset messed up the export.
The same when I printed to PostScript, distilled to PDF/X-1a.
What also failed: An export to EPS and then using the EPS in the layout.
The final PDF for the printers was messed up.
( Adobe Community Expert )