The test for verifying that two simultaneous compactStructuredEncryptionData commands on the same encrypted collection are serialized assumes that the second compact command that is sent from a parallel shell will block until the first compact operation finishes. This is true for the replica set version of this command, but not for the sharded version. The sharded version of this command is performed by a CompactStructuredEncryptionDataCoordinator instance, which is a primary only service; if the first compact request is running on an instance, and the second compact request is on the same collection as the first compact, then the second compact request is "joined" with the same coordinator instance and will return the same reply as the first compact request.