In a standby cluster, the injector node acts as the replica set "primary," but it can only serve a subset of replication-related commands (e.g., hello, oplog reads). For all other commands, it returns an injector-specific error. This causes issues for mongoS and other cluster components (shards/config servers) that prefer talking to the primary during startup and normal operations.

Solution:

Add an exclusion in the RSM ServerSelector to filter out nodes tagged with

 {"processType": "INJECTOR"}

from consideration when targeting nodes for non-replication commands. This ensures replication continues to work normally while preventing other components from inadvertently routing requests to the injector.