Wrong modifer is assigned during Price Adjustment

The requirement is to overwrite the selling price in the customer order. I am using Order Management HTML UI. I have created 2 modifiers with Discount and Surcharge, application method is Amount. When I overwrite the price, a modifier is automatically assigned. The problem is when I increase the price, the modifier for surcharge is assigned which is correct. But if I select a lower price, the modifier for Surcharge is also assigned instead of Discount.

My expectation is that depending on the amount Discount or Surcharge Modifier is allocated.

Is it because of the HTML page? Do I always have to select the modifier first and then adjust the price?