As long as I know, something wrong with his brain more than other family member. Ashley (his father) also can't do anything about it. He's so confused how to train his second son. So, to prevent him killing someone (bcs his moral compass is so broken), Ashley often lock him up. He's not a bad guy but he can't tell if something is bad or wrong.
First thing first, keep in mind that the one who's thinking it was an abuse is Dane... Grayson didn't think that moment as a type of abuse, but rather a consequence for doing something wrong.
This "get locked in a room when i was a kid" moment, comes from Ashley's method of disciplining his extreme alpha's sons. In the end of the story, Ashley will tell Dane more about the backstory of this method, so we will eventually know anyways. (I can spoil you the backstory if you want)
But to summarize:
Miller family is complex. Being an extreme alpha means you are prone to experience a "pheromone frenzy" which makes your head and everyone else around you go crazy, furthermore it's also expected for an Extreme Alpha to born without sympathy. Now, imagine you have 4 kids who managed to manifest as an Extreme Alpha. Even one of them (Grayson) is an extreme alpha since birth.
Ashley comes out with that method of disciplining because of the nature of his extreme alpha kids who's born without empathy (except for Chase). And like we've been hinted in the story, no normal way of disciplining can be applied to those born-with-empathy, extreme alpha sons. So the extreme way it is.
Ashley might not abuse his kids physical way, and yes... he and Koi (especially Koi) is spoiling them. But Ashley's parenting is unusual and never normal (like this one, and on other case: when he told Nathan, Grayson, and Chase to have intercourse simultaneously to release their pheromone—which later making Grayson "push" Chase to do it with Alex)
Ahh thank u so much for this, u took ur time and explained. It makes so much sense now.
My dumbass quite literally thought Grayson's father might be abusive and shii but also doesn't looked like he would abuse his kids at the same time so I was confused asf of what the "punishment" Grayson might have went through meant. I appreciate it twin

Guys, I haven't read the novel.
Can y'all tell me why Greyson, a rich kid would think like that?? Wasn't he spoiled from the getgo?
I'm confused