These fit your request I think:
https://www.mangago.me/read-manga/itsuka_no_koi_to_natsunohate/
https://www.mangago.me/read-manga/love_order_1/
https://www.mangago.me/read-manga/fuck_buddy/
https://www.mangago.me/read-manga/star_struck/ (the most red flag from what I remember)
https://www.mangago.me/read-manga/meguro_to_akino_wa_kizukanai/
https://www.mangago.me/read-manga/dog_and_bird/
(not all of them are necessarily red flags though, some are yellowish or are just oblivious)
Thank you and i watched the series of star struck but didn't read the manga . I will read
Is there any manhwa where the top and bottom are best friend but the top is the red flag, always give mixed signals, date others but not the bottom and after broke up with others come to bottom.