What if the reason why Taeyeon was in a good mood going to BKK was bec there was no need to cover up for KF anymore, and maybe Taeng also wanted to get all the attention on her to blindside people from noticing Tiffany going to Cali. Everything seemed so planned, perfectly, until someone saw Tiff in Cali and of course saw Khun after. Maybe, maybe the reason why Taeng was so emo before was bec she had to cover for her BFF and sometimes  really thinks that " when do i get to cover my own  rel" it was always for someone else but not hers.although i believe in Taeny, and that they have a deeper bond between them..but honestly we don't know what's happening behind the camera, the interviews, etc., i actually shipped them romantically but always had a feeling that Tiff was already taken. And honestly, personally, if I have to cover everytime for my BFF so her relationship wouldnt be out in the shadow..i'll be a littlr bit emo too, at the end of the day, she'll say..when's my turn? 

also that i think Tayeon is worried at the same time happy too for Tiffany....she's her best friend...it's by default.....no offense..just a plot twist ;)