Missing content with pdf version 1.5

I’m using PDF.js Viewer 2.1.7 in a wordpress site with generally excellent results. But when viewing a pdf version 1.5 file some of the content does not appear (firefox 104.0.1 Linux). The file appears fine using Evince desktop viewer on same machine.
If I convert the file to pdf 1.3 (not 1.4) using ghostscript then the content is all there, but text quality is degraded (somewhat washed out); again the file looks fine using Evince.
With either version 1.3 or 1.5 no warning or error appears in js console.

The website is password-protected but I can provide the file in question as well as screenshots if you have an email address I could send it too.
Thank you!