Some airlines don't treat printed boarding passes with the same "weight" as others. On both SQ and EK, I have pre-printed my boarding pass, and still had to check in, and got a proper boarding pass, so it seemed to me that the process was kinda worthless, apart from being able to pick my seat early.

FWIW, I have also had problems checking in online with TK generally (rejected two out of three times, requiring me to check in at the airport).... You might want to check if ICN is within the permitted airports for TK online checkin? (Don't even know if this is still a thing...)

None of these were transfer though, and I was always travelling with checked luggage.