How do I set the default card for Apple Pay on macOS?
This can happen when you have Apple Pay enabled on your Watch with a different payment option as the default there. There appears to be no way to configure the order in which devices are offered when more than one (Watch and iPhone, for example) are present, so that if Watch is present (unless Watch is disabled for Apple Pay entirely) it will "jump the queue" and offer whatever is the default there, independently of what you have set as your default on the iPhone.

To prevent this from happening and see your iOS-configured default card on macOS, disable Apple Pay on your watch (Watch > Wallet &amp; Apple Pay, and remove all cards there, including Cash; turning off "Allow Payments on Mac" surprisingly doesn't do the trick).