Port forwarding requires a port forward rule that matches the inbound connection. If there's no such rule... NAT won't stop the connection, it will just ignore it.
If no other aspect of your setup blocks the connection, it'll be successful. If you were deploying NAT because you thought it would function as a firewall then this part is probably not intentional.
Yeah that's called port forwarding. It is like complaining that light is coming into your house through windows. Fully intentional.