Its a good and bad kind of annoyed
omg is it the drug scandal, moonda was worried for?
I don't think it's fair to call it "instinct" since instinct makes it sound like a skill or something. Big sejin is right with the neglecting health issue but he is also 100% projecting his past issues unto small sejin. He didn't like small sejin even during idol inc days and none of that was small sejin's fault
He's also very desperate to keep Testar afloat since he stayed a trainee until he got sent to the survival show and finally debuted with the others. In addition, it doesn't help that actor Sejin is very closed off when it comes to his problems and is stubborn.
Big sejin is annoyed with smol baby sejin bcoz hes neglecting his health and all. He's also annoyed bcoz he's (smol sejin) hiding something (if my memory serves me right) that will affect the grp. Big sejin have this instinct (〜￣△￣)〜