they didn’t forget ahaha i think the uncle man released all the shackles because we’ve seen a panel where one got released so im assuming that all of them got released. Because the man also said he wanted s*x and if sihyeon is all tied up then it would be masturbation instead of s*x
actually, there was one panel where they forgot to draw them. It was before the scene you are talking about (so in earlier part of the chapter), when Sihyeon had his arms behind his back in a weird twisted position that would have make zero sense if it were not meant to be with bound wrists
Quick Sihyeon! The artist forgot to draw the shackles on your wrist! Run for it!