Why is the salary record not getting updated?

We have noticed some inconsistencies with the Salary form when changing an employee's grade.

If an employee's assignment is updated with a grade change from, for example, 01.SW.4. to 01.JBT.4. on the 01-AUG-2016 the salary record does not reflect this change with an update on the salary record if the rate does not change. The grade will remain as 01.SW.4. on the salary record until there is an actual grade rate change involved. This means that the assignment grade and the salary record grade are not in line.

Is this expected functionality? Have we missed a trick somewhere? We are using the grade ladder functionality, this issue still occurs even when there is a change in grade ladder but not rate.