Maybe you're not taking into consideration that being an omega is actually very new to him. When he denies Taehan, it's not really because he's a tsundere but he's having anxiety attacks (like that one time they were at the other omega's house)
Since Taehan's pheromones makes him calmer, you don't recognize him anymore
The author is a big vague on the timeline, but from my guess, it’s been less than 2 weeks since Doyun started experiencing his heat.
So while it’s been 30 chapters and couple of months for us, in reality, Doyun is experiencing so much change in a short amount of time. He’s actually adjusting and adapting far better than most people do in his circumstance
I love Doyun so much, and have many feels for him, but I don't understand why he bothers me a little bit? What does he do that makes me not understand him?