> FWIW, I canʼt reproduce this. Which distribution is this on?

This is on Arch Linux where the JetBrainsMono font is installed as a
system package in version 1.0.1.  I've seen that the current version is
1.0.2, so maybe the issue is fixed upstream alread...

So I've just downloaded the JetBrainsMono-1.0.2.zip and copied over the
new TTF files to /usr/share/fonts/TTF/ overwriting the existing ones and
ran "sudo fc-cache".  Therafter, my emacs shows mostly boxes: