Workflow inconsistencies: Collect Feedback 2010 WF on SharePoint Server 2013

I am trying to troubleshoot inconsistencies on an out of the box 'Collect Feedback 2010' workflow running on SharePoint Server 2013. Task assignees mark their tasks as complete - sometimes this is reflected in the Workflow History but not in Task List, and sometimes in the Task List but not the History. The visual representation of the workflow doesn't always show the green check mark on users who have marked their tasks as complete either. The result being users getting 'overdue task' emails for items they've completed.

Has anyone had this happen before? The WF is so simple, I'm wondering if it's a server issue. Not sure if this matters, but the WF tasks are being assigned to a Security Group that has other Groups nested inside.

Thanks in advance for any ideas here.