The 'z' field is not part of the schema for the initially inserted documents. In this situation typeof doc.z === "undefined". And it is known for {multi: true} updates to entirely miss updating a document due to an interleaved chunk migration. I think the appropriate change to make to the update_multifield_multiupdate.js FSM workload would be to relax the assertion about the 'z' field being present in the document when TestData.runningWithBalancer === true.