buildFHSUserEnv does not fully honor fonts in configuration.nix?

CJK and emoji characters were not correctly displayed when using joplin-desktop and devdocs-desktop, looking up in nix store, both /nix/store/<hash>-<name-version>-fhs/etc/fonts point to fontconfig package, but some important settings like default fonts are under different store path and the fhs didn’t reference them.
Is this a package call site misuse, buildFHSUserEnv bug or just how it is ?

Workaround: make an user local font conf as fontconfig has a 50-user.conf.