Relationship between 1 or more fields in Sales Opportunities section

When I try to change the BU (Business Unit) on an Opportunity already configured previously, the system automatically updates, bringing to the lowest value, 2 fields present in the Opportunities screen.

Attached are two screenshots, before and after, of the unexpected behavior.

The customer expects to be able to change the BU but without the "Sales Stage" and "Win Probability" fields changing.