Responses
According to his military enlistment record, he is really a BETA. But along his sexual encounters with ML, MC's body gradually changes. However, I'm on the 99th Chapter of the novel, and the fact that MC emits pheromones that only the ML can smell since their first encounter, still doesn't have an explanation.

In the novel, ML suddenly panicked after realizing that he had indeed knotted inside MC. He has been a Golden Alpha throughout his life, known for his immense sexual control. He actually hated the fact that alphas are acting dominant over other genders by using their pheromones, and strictly trained himself not to release his own pheromones in any of his previous sexual encounters. He was utterly disciplined about it, always in control.
Yet since his encounter with UKE, ML has felt like his control could explode at any time. That's why in the first few chapters, he was acting rude toward the UKE but completely curious about his scent, which was a stark contrast to what UKE always claimed that he was a BETA.
In this chapter, ML finally lost himself in the midst of pleasure. Not even aware of what he was doing to the MC, like releasing his pheromones and even to the point of knotting, something he never did before. In the novel, he literally PANIC, and then scraped his semen out of MC's hole, haha. He only calmed down after MC comforted him, telling him he was a BETA, that he couldn't get pregnant.
And sooo...spoiler stopped here.
P.S. Don't worry, guys. MC didn't mind the ML knotting him. In fact, he loves it that their following sexual encounters can't be without KNOTTING. He demands to be knotted. Like twice or even thrice. I'm telling you, they are both freaky when it comes to lovemaking. One time, they even filmed themselves having sex.