I am unable to make changes to assignment finish dates. When I manually adjust the finish dates on the assignment, the date reverts back after saving. I am only able to change the finish date and have the new date stick when adding ETC hours to that assignment.
This is working as designed. When an assignment has no work (ETC hours) remaining, PPM assumes that the assignment was finished on the date that the last actuals were posted. This date will be the assignment's finish date, unless they are provided more work (ETC hours) to complete. This behavior will occur regardless if a task is fixed duration task.