Hello, I believe this issue had been fixed by:
commit 6f12880a76a6c91058718d15845f8b9d9b529a5a
Author: Sarah Morgensen <iskar...@mgsn.dev>
Date: Fri Jun 18 17:52:05 2021 -0700
gnu: fontconfig: Use (locally) deterministic caching
Make fontconfig use directory contents rather than modification time to
determine cache validity (by pretending that mtime is broken).
Closing.
