Frequently when I run provisioning tasks some of the machines will disappear from the task. The task may or may not finish successfully, Has anyone else seen this behavior? The provisioning history will either says all the tasks were successful or nothing shows up. This doesn't seem to happen with software distribution, only with provisioning.
Are you adding machines to a task via a query? The only time I've directly witnessed the behavior you're describing is when machines are added via a query, and after being provisioned, the device is no longer a result of the query, so the next iteration of the task will remove it from the list of machines.