I don't believe the problem is with fontconfig per se - there is a startup script invoking fc-cache in the fontconfig extensions in x86, x86_64 and armv7.
I don't really see why loading dejavu-fonts-ttf would fix the problem in x86 when luxisr ttf fonts are included in the Xlibs extension anyway and dejavu-fonts-ttf are not required in X86_64.
There is a circular dependency between freetype and harfbuzz in X86_64 that is not present in x86 to avoid bloat, but I don't see how this would affect things.
..and I also see that the error: W [16/Jan/2018:17:34:04 +0000] [Job 1] Ignored bad font "FreeMono"
E [16/Jan/2018:17:34:04 +0000] [Job 1] No usable font available
..occurs with x86 and Xvesa, but not with x86 and Xorg-7.7...
Edit: with Xvesa and Xorg-fonts, cups also works