Filing Activity on Case gives inconsistent results/display

To reproduce:

• Create a phone call activity for a contact
• Create a Housing Support Case for the same contact
• Note the ID of the activity
• File the phone call activity on the case
• Look at the activity in the activities tab and the activities listed on the manage case page (hover over the edit links to note the activity ids)
• Note that the Activity ID for the phone call has not been incremented in the activities tab but the subject has been update on this older activity with "Filed on Case XX" appended
• Note that the Activity ID for the phone call has been incremented in the Manage Case activities list. This is the newer version of the activity. It does not have the the subject 'Filed on Case XX" appended.

The result is that the user has an edit link that points to different versions of the same activity. Both can be edited independently.