Placed vector graphics show as low-resolution previews on export of book to pdf
In InDesign 14.0.2 on macOS 10.14.6, when an InDesign indd file containing placed vector graphics (PDF or Illustrator files) is part of an InDesign book, and the entire book is exported to PDF, some graphics in the PDF show as low-resolution previews.
Steps to reproduce the issue:
Create two or more InDesign files with placed vector PDFs, and save and close them.
Add these InDesign files to an InDesign Book file.
Export the Book as a PDF.
Placed vector PDFs show as vector graphics in the exported PDF.
Some placed vector PDFs show as low-resolution previews in the exported PDF.
See attached file. Note that in the PDF the bar chart on the second page is a low-resolution preview, even though the file is correctly linked in the source InDesign file. This PDF export was made from the InDesign book file with both InDesign files closed.