Hi Derek,
I have two rendering issues with xpdf, and the 2nd of which also affects every other viewers on Linux I have - mupdf, acroread 9.x , evince. But the file works correctly on acrobat reader on android, so it seems to be a rendering problem, and perhaps generic to Linux-based viewers - for the 2nd issue.
The file is http://htl10.users.sourceforge.net/FontVal-TypoLabs2018.pdf . Sorry it is a 17MB file - processing the eps alone does not trigger the problems.
Page 73 - the Devanagari accents, the dotted circles shows only 3 dots (should be about 8). Page 72 has a figure from png screen capture showing what it should look like.
Page 76 - the Arabic glyphs are not positioned correctly and so the writing is a bit broken up. Again page 75 has figures show what it should look like. This problem affects every viewer I find on Linux also - mupdf, evince, acroread 9.x .
The figures were generated as eps's from harfbuzz / cairo with embedded fonts, then passes then latex and drips then Ghostscript. I first suspect a generic issue with one of the generating tools as the 2nd issue affect every viewer on Linux (the first issue is xpdf specific). But current acrobat reader on android display the expected result.
I tried also the official binaries (both 32-bit and 64-bit) from the xpdf website, just in case. (my system is mostly fedora 28, but the 32-bit freetype is 2.9.1 as is from fedora 29, while the 64-bit freetype is based on fedora 29's but modified with the current fontval patch)
I also have a small complaint about the new 4.0 GUI - the navigation forward/backward buttons don't do next/previous page, but does some kind of history navigation - e.g. If you do page 1, then jump to page 50 with the explicit page number input, then press the backward button, it goes back to page 1 rather than page 49. Also "n" and "p" don't work like the 3.0 motif/lesstif GUI... So navigation is a bit painful with the 4.0 GUI.
Hin-Tak