Vue.js How to mirror input field with v-model and computed property and a checkbox

I am trying to figure out how to properly mirror two input fields and dynamically change them based on a checkbox value. This is the best what I can think of using computed property with get and set features.
The problem is when the user types (1)”ABC” in shipping details, (2)unchecks (copy_chekcbox=false), (3)types “123” in billing details, and (4)checks back (copy_chekcbox=true) (5)the billing details doesn’t revert to shipping details, which is what I need.
Image with steps