Unfortunately, there were multiple regressions when running apps from unstable on 20.03. They should be fixed now but if you encountered the issue your font cache was corrupted and you need to delete ~/.cache/fontconfig.
You should not need to run fc-cache -r as the NixOS module already generates a fontconfig cache. You will only need it when running apps built against incompatible fontconfig versions but you would need to run fc-cache from the same fontconfig package that the programs are linked against and that is just too annoying IMO. Instead I would recommend just updating 20.03 and unstable channels to the latest respective revision and deleting the cache directory.