I think you feel like that because everything is ambiguous at this point.
The older brother seems to be depicted as obsessed with Haru from the bit we've seen so far and the little brother's perspective, we still didn't get the whole story from his point of view or the content of the letter, so we'll see about him. His words about being an alpha for the purpose of protecting and loving him the most at the end of chap 4 were forward about his sentiments, but the vibe of the chapter was strange...
The younger brother seems to genuinely love Haru, but became indecisive because other factors like his sense of inferiority towards his older brother and the way the older brother showed a provocative attitude about Haru's preference toward him. I feel like his attitude during his brother's absence was unknowingly punishing Haru for only seeing and caring more about his brother; he has room for improvement as the story goes on though. If he starts to be true to his feelings and treats Haru better, he can still become main male lead.
Haru seems to have been more attracted to the older brother from the start, as he was the one to protect him and give him a sense of security. However, during the long absence he seems to have developed feelings for younger brother. The fact that younger brother prioritized his fiance seems to have hurt him deeply. But the reason I am not sure about which one he really loves is ; although he showed preference for the older brother from the start and seems to still like him, he reacted very strongly to hearing about the younger brother not caring about him, and he only took off the collar after believing it, rather than for the older brother.
I think he is unclear about his feelings for both, but since the older is being more outwardly loving and gives him a sense of security and if being his number one, he is choosing him right now.
So it could still change later depending on the attitude of both brothers, if the younger fights for his love and wether or not the older is really in love with him.
The uncertainty about who is the real male lead makes it interesting as it can go both ways, but I do feel like it can also end badly, or at least with him walking from both of them.
I find this story kinda a slog to read because no one seems to really... Like each other all that much.
I mean, they'll give lofty speeches about "love" , but it seems kinda hollow. Like they just like the concept of "love" and possesion.
I'm not sure if this intentional on the part of the author or not.