Like I said, it's sending it to the email address that you've got set up as the family organiser which is probably the same one as your apple id email address.
I'm pretty sure if you go into imessage on the mac as I said, and remove it as a way of being contacted for messages, and go into the same on the phone, as I said and make sure it's there in the imessage settings, then you'll have what you want.
But it hasn't decided anything different. It's sending messages to that address just the same as it always has and if you have it set up to receive messages on 2 devices it'll send it on 2 devices. Or three.