I do believe there was bullying. Even Naeun's mother was mean to Hyunjoo on TV (!) and Naeun's sister got rumours too that she bullied others. From the way they treated and talked about her despite knowing that Hyunjoo attempted suicide it can be quite obvious imo that they bullied her. A normal human being wouldn't say that the person who tried to kill themselves is weak, especially not so openly, even if you dislike the person. A basic amount of empathy should make you at least second-guess if maybe yourself said or did something which wasn't nice (even if you did nothing wrong) and will make you shut up at least openly (to not harm the person further AND to protect one's own image by not being seen as the shameless cold person making fun of someone who tried to kill themselves).
Naeun's family, the diary entry of Naeun and the way April reacted to knowing Hyunjoo tried to committed suicide don't make me doubt that they definitely tried to make her feel bad on purpose. If you add the tumbler and the shoes incidents and that Hyunjoo was really popular on the show The Unit and her group members of that project group all really like her even until now, I doubt Hyunjoo was the problem. I do think it escalated because they were jealous she was allowed to go home a lot and from then on one or two members started being mean and the other ones followed and they all bounded over having a common enemy they feel that doesn't deserve to have the "privileges".
Naeun also profits a lot of a PR campaign right now. They try to get her back in the spotlight in a positive light since a year now. They even paid for her to attend a youtube show of a guy who (it seems like, I don't know him) was bullied and spoke up against bullying a lot and basically tried to make her seen like she wasn't a bully. Well, that guy lost his credibility and had to apologise for this. And all the things coming out in favour of April (and especially point out how good Naeun was to her members) suddenly and about for example staff members (I think it was a make up artist?) who shields April but at the same time talks like she was 24/7 there and knew what was going on even in their dorm etc. is just unbelievable. Naeun is under contract now of a famous actor agency and they try very hard to get her seen in a good light, so I wouldn't trust these random stories after all the things they literally showed themselves how they saw and treated Hyunjoo. April lost all 7 lawsuits and with every statement, where they wanted to defend themselves or get anyone else to defend them, they made themselves look even worse. I don't think I have ever seen a group looking worse each time they try to defend themselves as I saw with April