Dunn is a playground player - he's spent his whole life making shots while on the move. It's become such second nature to him that he isn't comfortable shooting while standing still. He will take the exact same shot from the exact same spot if he can dribble to it that he won't take if he is standing there all by himself and gets the ball.
He's clearly worked on the midrange jumper though, which is good. It's the same 2 points whether he shoots it on the move or standing still.
Cole, Greg, Champ, and to some extent Earlington can all hit the spot-up three, so there's enough to work with there from an offense standpoint, even though it's a handicap for Dunn.
The flip side is Moore, who is in love with his standstill 3 and at this point should probably never take it. He has a great midrange game, but he's (at least) a year behind Champagnie (who has extended his range from the mid-range last year to the 3-pointer this year).