Does this problem exist in the latest "web app" version of PDF.js or only in the built-in viewer, which may be a bit behind? You can test the web app version here:

https://mozilla.github.io/pdf.js/web/viewer.html

Use the "Open file" icon on the black toolbar (next to the print icon) to load your PDF from disk for testing. This is handled in-tab and not submitted to the server.

I am now on a different Win10 PC, running 76.0, not a beta version. Running test.pdf from here using https://mozilla.github.io/pdf.js/web/viewer.html, I see a blank page, which is incorrect, but different from what I saw on the other Win10 PC. It is vaguely possible that the test.pdf files on the two PCs are different, being separately generated with possibly different versions of ghostview, but they display the same in Acrobat. The PostScript source is:

100 100 translate 10 setlinewidth 0 1 2 {/x exch def x setlinejoin

 0 1 2 {/y exch def y setlinecap
   100 x mul 100 y mul moveto closepath stroke
 } for

} for showpage

Sorry, I don't know what to do with PostScript code...

Do you want to post an example of a PDF that doesn't render correctly in beta on some storage site and share a link to it?

https://github.com/ms0/test/blob/master/test.pdf

The PostScript source is there as well (test.ps); I use ghostview to convert ps to pdf.

For what it's worth, it looks blank to me in both Firefox 76.0 and Nightly, in both the built-in viewer and the web app viewer. So maybe file an issue on pdf.js? https://github.com/mozilla/pdf.js/issues/

I have filed an issue in pdf.js, as you suggested.