If we forget about Nol's injury for a second, this is the second time he flinched when Shinae hugged him back.
Before anyone come at me with "No, he clearly flinched because his back is injured!", let's take a look at ep 151.
See that hand? His grip literally loosen the moment she touched him.
Which is why, I think, even without his injury, he'd still do the same thing.
It's apparent Shinae touched the sensitive part of him. It's almost symbolic. It's like he didn't anticipate someone will hug him back when he hugged them, Like, he did not expect someone will reciprocate his feelings. Or maybe he still has a trauma for being touched? Like there is still a part of him that reject the kindness someone show him.
Nol has a long, long way to go before he could heal.