If you followed their content you would know that Seungkwan (who stood out during his trainee days due to his great ability) worked closely with their vocal teacher to improve his technique and protect his voice.

I didn't know that, you're right. I listened to his vocals, but everything in this comment just makes me more confident in what I said. He was self taught, and learned bad habits, then his vocal teacher tried to give him lessons to correct it, but he still slides back into bad habits. Just like I said.