When you change the task finish date and save it, the date changes automatically which is not the date you had set nor the date which was in there before but altogether a new date.
Steps to Reproduce:
Expected Result: The finish date changes to 06/20/2015
Actual Result: The finish date changes to 08/16/2015
Release: All Supported
The primary reason for this behavior is the ACTUALS THRU (date) attribute.
You can remove the resource assignment and add it back. This will clear the Actuals Thru Date and you will be able to change the finish date.
The system won't let you remove the resource assignment if there are actuals assigned to the task. Here is what you can do: