The return value of the doCompactOperation is assigned to the member variable _response, which is in-memory state that is not persisted by the coordinator. Since this variable is being assigned in the penultimate kCompactStructuredEncryptionData phase of _runImpl, it is possible to lose this data if a stepdown occurs while in the middle of the final phase. And when the new primary resumes, it'll have an invalid _response value, which may cause an invariant failure.