When Korean fans go looking for returned tickets

Sold out is not the end. Seats held by bank transfer and never paid for are cancelled automatically at a fixed time, and those seats go back on sale.
The deadlines are published by the sellers themselves. YES24 Ticket: 23:29 the day after booking. Interpark (NOL): 23:59 the day after. Melon Ticket: 23:59:59 on the day of booking.
So Korean fans are already on the page five minutes before that, refreshing. The seats cannot come back until the deadline passes.
On Melon Ticket, fans report the wave arriving between 00:10 and 00:20. The deadline is the same time every day, so the cancellations clear together.
The week to three days before the show is the other window. That is when people who have changed their minds let the tickets go.
This site's cancel clock works the time out for each sold-out show. Turn the phone alert on and it tells you ten minutes before.