In my experience it's priority boarding at the first check at the door then down the ramp at C gates and filtered into Y bus there. I've never had a J bus when flying Y, and that's as Gold where there is no ambiguity about priority boarding.
I should say that is true for me too. It was only on the one instance where they were having folks wait for the special bus that it was offered. In that case I preferred to board immediately anyways. End result is I've also never had anything but the Y bus for Y tickets despite status. Not saying that it should be different either.