Short answer
HubSpot's KB: defaults apply only to payment links created after the setting is on; existing links aren't updated. Billing and shipping defaults live in checkout settings, payment methods in payment settings. Individual links can still override.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Edit older links one by one if they need the new defaults.
3. Related
Sales tax and payment link discounts.