"Billing same as" copies shipping fields not rendered on billing

Problem/Motivation

When a custom field has been added to a profile, but is only rendered/displayed on the Shipping, it is "copied" to Billing even if Billing does not render that field. This is slightly different from the scenario described (and worked-around) in #3191764: Prevent extra profile fields to be duplicated in the "My shipping is the same as my billing info" fieldset. → .

Steps to reproduce

1. Add "Delivery instructions" field to config/people/profile-types/manage/customer/fields.
2. Under "Manage form display", expand "Custom display settings" and check both "Billing" and "Shipping". Save. This is the core difference between this issue and #3191764: Prevent extra profile fields to be duplicated in the "My shipping is the same as my billing info" fieldset. → . The other issue says "use a different profile type entirely" or "use code". This particular approach, though, is using the core-provided options for form display of a Profile.
3. Under "Manage form display", add "Delivery instructions" ONLY to the Shipping form display.
4. Under "Manage display", add the "Delivery instructions" field to the "Default" view mode.
5. Add a product and checkout. Under "Shipping information", fill in "Delivery instructions.
6. UNCHECK "My billing information is the same as my shipping information." The billing fields load in and, as configured, the "Delivery notes" field does not show up. So far so good.
7. CHECK "My billing information is the same as my shipping information."
8. Fill in your shipping and payment information. Then "continue to review".
9. "Delivery instructions" and the filled value show for both Shipping and Payment information.

Proposed resolution

If a field is not rendered in Billing, it should not be copied from Shipping.

OR:

Remove the ability to enable different "Billing" and "Shipping" custom display settings, so that people are steered to the "working" approach of #3191764: Prevent extra profile fields to be duplicated in the "My shipping is the same as my billing info" fieldset. → (i.e. use a different Profile type for Shipment types).