I think it had more to do with Luffy behaving like rubber despite DD using Haki, not Luffy himself using Haki. Him being surprised does not make much sense since he already knew Luffy is using rubber properties to a certain effect.
He says this before that scene when he attacks Luffy, meaning Luffy uses CoA on the rubber, then compresses it, and uses the tensile force of it bouncing back to increase his attack power, something he could not do to this extent without Hardening it first.
https://v217.mangabeast.com/manga/One-Piece/0784-014.png
So DD already knew there is interaction between CoA and Luffy´s rubber ability.
So i think this is referring to himself using Haki and Luffy still bouncing it back like a normal punch/ blunt damage.
https://v217.mangabeast.com/manga/One-Piece/0784-016.png
Regarding the two other instances, they were the first time he used the attacks, and Oda drew them in detail and step by step instead of drawing the attack in one panel, so that the audience can comprehend what´s happening.
And Red Hawk is a good example, because next time he uses it against DD, he already had Koka on his arm before stretching (in the middle panel on the left, arm is already black)
So Red Hawk was already ready here, the next panels were just illustrating Law switching himself and DD