Seems MS recently removed WorkflowDefinition.Properties.["SMLastModifiedDate"] from all SharePoint Online workflows. The closest substitute is WorkflowDefinition.Properties.["Definition.ModifiedDate"], but that property is a string expressed in the web's regional date and time format.
Depending on tenant size, dumping metadata can take hours. If during this time, the network or SharePoint experience an intermittent issue, the resulting .NET exception causes the dumping process to halt and it must be restarted from scratch. To prevent this, allow for some number of retries until giving up. With each retry wait for exponentially longer to allow for the network server to recover.