Thanks a ton for the response & explanation! So I guess I will definitely have to go landside via transit visa, change terminals and then check in again at the Asiana counters. Time shouldn't be an issue, I got 6 hours in Beijing.
My bag is definitely checked all the way to Busan, it's correctly written like that on the baggage tag I got. It feels super odd though that my luggage gets a ticket for both legs, while I can't...