Symptoms
So I have a process that takes an input of a People Picker from SharePoint, copies that to a Process Data field and then assigns that Process Data field doing a Split function on it by the delimiter of a pipe to assign to the Plan Just Once slot of a Task. However, the problem is that if there is more than one person in the field, only the first person ever gets assigned. If there are 2 or 50 people, those other people don't ever get assigned the task and don't get the notification.
Diagnoses
When using the Plan per slot (no destinations) planning option, make sure to add a Succeeding Rule stating [ActivityDestinationInstance_Status] All = "Completed"
Resolution
Adding Succeeding Rule stating [ActivityDestinationInstance_Status] All = "Completed" to "Set Reviewers" activity
Per this KB, http://help.k2.com/kb000685, and redeploy your process resolved the issue.