The Membership Dues template is designed to hinge on the 'Bill to' field. This field causes the template logic to find any membership associated with the selected constituent. When you update the constituent, the logic fires and as a result refreshes all of the fields which depend on the 'Bill to' field.
Working as designed.
Steps to Duplicate
1. Create a new Membership Dues batch. 2. Add a constituent in the 'Bill to' field. 3. Complete other fields in the row. 4. Return to the 'Bill to' field and select a different constituent. 5. Note that all the fields which depend on the 'Bill to' field have been reset and now offer new values based on the selected constituent.