The changes from facdcf1 as part of SERVER-38810 introduced a transcription error. Prior to those changes, a ConflictingOperationInProgress exception would lead to the session info for retryable writes being skipped due to the recipient of the chunk migration having a higher txnNumber than the donor. A ConfigurationInProgress error is only used the replSetReconfig command.

There doesn't appear to be a way for TransactionParticipant::beginOrContinue(autocommit == boost::none) to throw a ConflictingOperationInProgress exception that I don't believe this issue is causing any chunk migrations to fail. Chunk migrations on the 4.2 branch appear to be similarly non-impacted.