In my opinion, these flashbacks are Sumin's. I think Sumin saw a cotton candy stand at the amusement park that reminds her about the time when she was with Seju. Same with when Sumin saw Sungji putting sugar in her coffee a few chapters ago, she was reminded how Seju likes her coffee without sugar. So, there's a possibility that Sumin's
comparing the time she spend with Sungji, with the time when she and Seju were still together (unconciously?). But that's just my opinion.