https://docs.mongodb.com/manual/reference/command/setFeatureCompatibilityVersion/ states that setting FCV is done on a mongos, but reading the FCV back on a mongos is not allowed.
> For a sharded cluster, run the command on a mongos instance.
> The operation is undefined on the mongos instances. For a sharded cluster that has access control enabled, to run the command against a member of the shard replica set, you must connect to the member as a shard local user.
Why is mongos able to write FCV but not read it back?