Hi @Vijay_Rajpurohit and welcome to the MongoDB community forum!!
The MongoDB version 4.0 is a very old version and RC indicating Release Candidate (but not final release). I would recommend you to upgrade to the latest version 4.0 release which is 4.0.28 or to a supported version 4.2 or newer for major bug fixes and improvements.
Also, please note that, the minor upgrade (4.0.x) do not introduce any backward compatibility changes.
Config servers store essential metadata for a sharded cluster, including which shards own ranges of data for sharded collections. If something happens to their single config server their recovery path will likely be restoring the entire sharded cluster from a backup. For a production environment specifically, the recommendation would be to have multiple config servers to handle failures if any.
Since it is possible that you’re seeing the effect of a fixed issue due to the version you’re using, upgrading the server to the latest supported version would be my first step. After the upgrade, if the issue persists, could you share the output for rs.status() and sh.status() from the primary members of the replica set and mongos respectively.
Best Regards
Aasawari