Secondment position, issue with entry grade

Summary:

Entry Grade is not getting defaulted when a position has multiple Valid Grade

Content (required):

We assign temporary position during secondment to Employees to cover employees who are on long leave. But sometimes secondees have a different grade than the position being assigned. So we add Valid Grade to the position(it will then have two grades). But issue we are facing is system changes the grade of the first person's(who is on long leave) assignment record as well with the value from Valid grade. Is there a way to handle this?

Version (include the version you are using, if applicable):