I hope so, but alas I know nothing about fc-cache or related /var/cache/fontconfig
I note as things stand that running fc-cache -r -v results in some caches failing to write and final fc-cache 'failed' message.
As quick test (no doubt destructive - so not recommended unless person knows what they are doing), I did:
rm -rf /usr/share/fonts/X11/cyrillic
rm -rf /usr/share/fonts/X11/util
rm -rf /usr/share/fonts/X11/encodings
since these were the ones fc-cache was reporting as failing to write.
Thereafter, fc-cache -r -v
worked with what remained in /usr/share/fonts
and gave final 'succeeded' message
I have not idea why the above three seemed to not write