Why am I getting a User Cannot be found error?

I'm migrating a Sharepoint 2013 Enterprise Wiki to Office 365. The source has been through several domain migrations and we have hundreds of different users and many don't exist anymore. We have a mapping set up for our current users, and we configured an unknown user mapping to an Office 365 account.

When doing a Copy Subsite, I'm getting thousands of errors below. Shouldn't the unknown user mapping handle the case where the user cannot be found? If I run an Incremental migration after the initial migration completes, do you think the errors will go away? Or is there an option to ignore the source item's created by/modified by, and replace them with a different, valid destination user? I'm fine with not maintaining this metadata.

Thanks!

Max

Error: An error occurred during the operation of a service method: User cannot be found.

Stack:  at Microsoft.SharePoint.Client.ClientRequest.ProcessResponseStream(Stream responseStream)

  at Microsoft.SharePoint.Client.ClientRequest.ProcessResponse()

  at Metalogix.SharePoint.Adapters.CSOM2013.ClientContextExtension.ExecuteQueryWithIncrementalRetry(ClientRuntimeContext context, Int32 retryCount, Int32 delay)

  at Metalogix.SharePoint.Adapters.CSOM2013.CSOMAdapter.CallExecuteQuery(ClientRuntimeContext ctx, SharePointAdapter callingAdapter)

  at Metalogix.SharePoint.Adapters.CSOM2013.CSOMAdapter.GetFileProperties(String sServerRelativeUrl)

  at Metalogix.SharePoint.Adapters.CSOM2013.CSOMAdapter.GetListItemVersionFileInformation(List`1 fieldList, XmlNode xmlItemNode, VersionDataTrackingTable versionDataTable)

  at Metalogix.SharePoint.Adapters.CSOM2013.CSOMAdapter.GetListItemVersions(String listID, Int32 itemID, String fieldsXml, String configurationXml)

  at SyncInvokeGetListItemVersions(Object , Object[] , Object[] )

  at System.ServiceModel.Dispatcher.SyncMethodInvoker.Invoke(Object instance, Object[] inputs, Object[]& outputs)

  at System.ServiceModel.Dispatcher.DispatchOperationRuntime.InvokeBegin(MessageRpc& rpc)

  at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage5(MessageRpc& rpc)

  at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage11(MessageRpc& rpc)

  at System.ServiceModel.Dispatcher.MessageRpc.Process(Boolean isOperationContextSet)

Type: Metalogix.SharePoint.Adapters.ServiceError

HResult:-2146233088