The cache version has been bumped from 7 to 8 in fontconfig 2.14[1].  If
fontconfig is upgraded before libfontconfig1, fc-cache in the postinst
will still use cache version 7, and a subsequent upgrade of
libfontconfig1 renders the global cache in /var/cache/fontconfig useless
until fontconfig is reinstalled.

It seems to me that a stricter dependency of fontconfig on
libfontconfig1 seems to be desirable to avoid such a situation.