I notice on the web there are two distinct issues affecting Fontconfig 2.18.1 (in nixpkgs-unstable since around July 9). Just a heads up if you are also affected.
The first issue is your default generic family (e.g. sans, sans-serif, monospace) may not be correctly applied in your application (e.g. Chrome), due to a change in the font matching algorithm. This issue has been reported by both NixOS and Arch users and to the upstream. A potential fix will appear in Fontconfig 2.18.2.
The second issue is that some apps (e.g. Ghostty) may have issues finding fonts or even crash (1, 2) when Fontconfig 2.18.1 reads an old cache. Again a potential fix will appear in Fontconfig 2.18.2.
If this Fontconfig update PR gets merged before the current staging-next cycle ends, the bugs should be fixed once the latter PR hits nixpkgs-unstable. Otherwise we will have to wait for another month for the next staging-next cycle.
Normally I would just advocate for PRs be merged using this Discourse discussion thread, but somehow I have used up my three chances to reply to that thread, and I cannot reply to that thread anymore as my Discourse account is considered new (despite being 13-month-old already).