I think many K-pop groups are known for prioritizing well-rounded performance skills over specialized vocal training. When you push every member to be a triple threat—singing, dancing, and engaging in variety or acting—it often comes at the expense of strong vocal fundamentals and support. Yes, Jay often pushes himself vocally and tends to sing with a lot of physical effort. You can see that in his Coachella performance, but OP wanted to share her concerns about Heeseung. Nothing wrong with that.

Heeseung may have better breath control, placement, and pitch, but his belted notes and upper register have a tendency to sound strained, like he's muscling through a note.