All I've had time to test against is Kobo Nickel, but based on my results above and DNSB's report that the font checksum and name are identical, it's sounding like this may be a problem with the rendering engine.
However that guess is questionable based on the fact that some fonts seem to work correctly.
It's tempting to troubleshoot and chase down problems, but I already do that all day in a different realm. What I want from my Kobo Glo is a device that properly displays well-formatted ebooks while remembering my reading location; 2.3.1 does neither.
So it's
back to 2.1.5 with
ikarus9999's plugin for me.