I could be wrong but maybe it happened off screen? I don't recall them showing her much at all other than a couple of times and it was to point out her copying Yeonhwa's look on a consistent basis. Like when the blondie in this chapter (Narin?) said in a previous chapter that the girl was profiting off of looking like Yeonhwa... can't remember if she said she got sponsors... but who knows maybe she was stealing this entire time and not actually getting sponsored the items, like it could've just been a lie she told her followers online, meanwhile stealing from Yeonhwa's people.
I'm sure they'll show it in flashbacks but back on chapter 38 they talked about how she's progressively copying her more and more. She started off by just copying her general look in her voice but now she's moved on to wearing things that she's only posted once. So this just seems like the next progression from there getting things she actually wore
The stylist told Yeonhwa that the things she prepared for her kept missing ever since 2 months ago. Turns out, the highschooler copy cat (who i believe is her step sister, since her dad remarried) is, assuming, part of staff or is able to sneak on the set and steal her stuff. She then claimed on her SNS that she gets brand deals now, which is massive bs, since all those items she pretends to own bc of brand deals were, one, actually intended for Yeonhwa by the brands that she is shooting ads for, two, stolen by her.
I didn't realize that high schooler was a stalker like thief lol besides chapter 41, when did she steal before?