How hilarious would it be if we find out in a later chapter that Fuyuki took that candid photo of Sayuri so she could play some 4D chess and put her into check by uploading that photo onto her Insta or Line or any other social media profile she has under the guise of "innocently uploading a cute photo of her and her friend hanging out after she came back from Korea", but in actuality she did it in response to Sayuri's formal war declaration of confessing to Shiki at the fireworks festival. She tricks her and the readers into thinking that her feelings are exactly what she tells Sayuri and the readers at the restaurant, when really we all know how she feels, even though she doesn't realize it, since this may be a romcom, its still published by Jump, and therefore has some shonen to it, further proven with the "I levelled up my techniques in Korea" comment and her battle pose while holding her make-up/weapons lmfao
Anyway, she plays innocent and unsure to Sayuri, but then after she does her make-up and pretty's her up, she sneaks that snapshot of her, and then uploads it to social media. Then the next phase of her plan is enacted by having the photo spread amongst the students because Fuyuki is such a people person that she pretty much has literally the entire school in her friends list, and because the entire student populace saw Fuyuki's "finally back in Japan" post, and said post being of Sayuri after Fuyuki uses her Korean make-up technique upgrade, everyone begins to think Sayuri is super hot, like that episode of South Park where the girls start using photoshop to edit their selfies the way the Kardashians do with all of their photos and not just their magazine photoshoots. Which then leads to lines of guys from the school, and even guys not from their school, to start confessing to her all day every day, thus leading to her being unable to get any time with Shiki, be it alone or in general, as Shiki's too much of a nice-guy/pure and innocent/soy boy beta cuck (lol jk)/docile dude that he would likely just leave it to her and not forcefully get involved, and this continues all the way through to, and even past, the fireworks festival, thereby causing Sauyri to completely miss the deadline she herself set for both herself and Fuyuki, meanwhile Fuyuki laughs her manical supervillain laugh as she hijacks Shiki and keeps him all to herself, even though she probably still wouldn't have worked out her feelings for him yet even though she could have used this time to do so since her attack would have been meant to buy herself time to sort through all that lol
On another note, I feel like if Fuyuki joins the fight just to compete with Sayuri after she called her out and challenged her by even setting the fireworks festival as the deadline/face-off, if Fuyuki hasn't sorted out her own feelings for Shiki by then, then it would only just hurt their relationship in the long-run, cause either Fuyuki will realize her feelings weren't romantic, and lead to her breaking up with Shiki, who clearly has some romantic feelings for her considering he cried when he learned she was leaving before learning it was only for two weeks, and even after learning that, he still came after her at the airport to give her her hairclip back, that shit is more than enough confirmation for romantic feelings and even Fuyuki herself used it as a test to begin with lol but point being, if she doesn't sort out her feelings for him before they end up dating, it could lead to disaster, cause she would end up dating him and ditching him after she realizes they weren't romantic, or just stay with him out of pride/stubborness/obligation and then end up resenting him and their relationship slowly fizzling out and slowly losing its soul and turning them into those couples/families that have been together since high school even though everyone wonders why they haven't just divorced or broken up by now since they clearly resent each other lmfao
Either way, I like my first theory better. Fuyuki for 4D Chess playing Supervillainess FTW lmfao