Currently the shardCollection command sends _configsvrShardCollection to the config server primary using Shard::runCommandWithFixedRetryAttempts which will only retry on retryable errors (like NotMaster or InterruptedDueToReplStateChange) up to 2 times. This can lead to failures in the continuous config server stepdown suite, if shardCollection is interrupted by stepdowns enough times. Changing shardCollection to use Shard::runCommand instead will cause it to retry on retryable errors until the command succeeds or the maxTimeMS limit is reached.

A lot of other mongos commands seem to use Shard::runCommandWithFixedRetryAttempts as well (like drop, addShard, and splitChunk), so it can also be investigated if it is worth changing them too.