This content should properly be moved to ./src/extensions/users-permissions/strapi-server.js and be rewritten according to: Plugins extension - Strapi Developer Docs
I think this could be working then.
And additionally it seems like some changes are scheduled for the Version 4.0.5 (in the high hopes of it being merged): Fix unable to populate User in Users-Permissions by iicdii · Pull Request #11960 · strapi/strapi · GitHub