Late Finish and Slack Calculation in Microsoft Project

Why does the Late Finish date for an in progress activity with negative slack no longer calculate using the backwards pass? On a task in this situation the Late Finish is displayed as being the same as the Early Finish and the Finish, but the slack is still negative as if calculated from a Late Finish date that does not match the Early Finish and the Finish.

The Late Finish date for unstared activities with negative slack is calculated using the backwards pass. What is the purpose of and calculation change that occurs to the Late Finish Date with an Actual Start on a negative slack activity?