ZQ is in love with Dad not JJ. He is the closest one to Dad among the 3. Their conversation always meaningful and he will slays dragon for Dad. During childhood he glued to JJ because he can see that he needs her approval so his Mom can marries Dad hence Dad will becomes his too. Throughout their childhood, he spoiled her while LX disciplined her. It's seems to be a subconscious thing just like he washed the clothes during the night. The same with his so called love to JJ right now, he subconsciously traded washing clothes during the night to marrying JJ.
The cake decorating scene where he hugged her from behind, he thought nothing of it and it came naturally UNTIL he remembered his friend's words. If he is in love with JJ, he will be get red and bother since the beginning NOT as an afterthought.