If the PM is the assignee that is having the Commit Date issue, go to their profile and uncheck "Send work I assign to myself to my Working On tab". When checked, this is auto-clicking "Work On It" for any task the PM is assigned and that is how you are getting the commit date.
Since we are now in Workfront Home where requests and work items are on the same list, this isn't really needed anymore. I'd just bulk edit all people with plan licenses and uncheck it :)