It also looks like people who have been to some US states (or are arriving from) as well as some other countries need to fill out the Qcode quarantine form:
https://qcode.kdca.go.kr/qco/cd/codeInput2.do
Since I’m connecting in Qatar where the Korean government lists MERS as a concern, I filled out the short form and got the QR code - will be curious to see if anyone asks for it.