They're playing a game, the same one they've played time and time again, and Tim fully intends to win.

Xichen's cheeks are flushed a pale pink and it suddenly occurs to Mingjue that Xichen isn't doing that fancy Lan technique of using his core to evaporate the alcohol before it can affect him. Which means Xichen wants to get drunk. Which means only bad things for Mingjue, generally. Good, but very bad things.