Right, that’s how it should work. Unfortunately that’s not how it actually works most of the time in consumer.
Many devices don’t provide an option in the UI to open an inbound port on IPv6. For example, the latest and most expensive Linksys gaming router blocks all inbound connections and there are no options for different behavior. It doesn’t support opening any ports for v6.
The most recent TP link device I tested for my dad doesn’t even have a firewall. If you know the global IP, you can connect to any port you want.