Wait hold up lol I just had to reread your argument and you said: “Since the seme suffered about not knowing about all this, it would have made more sense if he asked why the uke never told the truth. And if he would have been a bit angry for knowing about it only now, i think it would be ok.”
Hojun was upset that he didn’t tell him sooner. Heck, he wasn’t even mad, he’s just upset. Even Mr. Ahn said Hojun was upset. From your own words, then doesn’t this mean it’s “ok” lmao http://www.mangago.zone/read-manga/the_good_teacher/uu/ibx_chapter-34/pg-24/
Yes it's all good to be upset/angry about only knowing this now. But he didnt even ask questions about it. He was too upset to even ask questions that he had to leave first.
It felt out of place to me. Atleast ask the uke why he didn't tell him sooner. Then get mad and leave if the answer is disappointing.
And I’m telling you it’s not out of place because it’s perfectly reasonable and completely in character given the circumstances they’re in. The fact that the author brought it up is much better than Hojun not giving a shit after being told the truth. It’s believable and logical, and I fail to see how you find it stupid.

If a guy confessed to me and then we became a couple. And then one day he tells me he confessed as a dare from a game and then came to really like as he got to know me. I wouldnt care really. It was a lie, but it's not like i was betrayed. If he cheated on me because it was only a game at first, it would be another story. But otherwise theres no reason to have make it dramatic.
Also....the uke actually teached him....the guy was so bad, he kept being dumped despite having this godlike body and nice personnality. So why even complain. Thats completely stupid.