Tsubame seems have a her fear of people losing interest in her, a lasting effect that lingers from her previous relationship, in which her boyfriend cheated on her with Sarina, who was seemingly her best friend.
Because of this, she has trouble figuring out how she likes someone.
One could speculate she takes time to see how she likes someone to make sure she doesn't get too attached to them in case they lose interest with her, so that if they do, it'd hurt less. This is reinforced by her University choice, which is far away from Shuchiin.
From this, we can infer that Tsubame makes it a duty not to leave most relationships platonic, or less, hence why she is an Impossible Girl.
In the case of Ishigami, one could infer that she sees him as a little brother considering how she treats him prior to the Culture Festival. However, the Christmas Party leaves a different impression entirely. I believe Tsubame saw Miko Iino as a threat to her, evident in chapter 154, page 18, where she states, "You're always thinking about Miko-chan." One could sense a hint of jealousy in those lines, specifically from the word "always," as if she was asking why he never thought of her like that. This being one page away from Tsubame's lie to carry out her "discovery sex" plan leads me to infer the following:
Tsubame was on the fence about her friends' idea of fucking Ishigami before Iino began asserting her dominance by making quick jabs at her; the complicated Pina Colada order with clear distaste on her face intimidated her. Ishigami's behaviours when worrying about Iino further drove that intimidation that Ishigami was losing interest, fast, so, to bring him back, she had to go with the sex idea anyway.
The fact that Tsubame worries about losing Ishigami's attention hints a romantic interest in him also, although her thoughts are cloud this interest in him. Tsubame's love is budding just like Osaragi's was before it was shut down once she saw Ishi with the sutra.
