Garnacho still learning, if rather slowly. Unfortunately I think he's becoming more like Rashford match by match, however, maybe that is what he is told to do?
In his early matches, when receiving a pass, Garnacho would inevitable move to meet the ball, thereby dislodging his markers starting position to challenge him. Now he seems to want to wait until the ball reaches is feet, defenders (especially fullbacks) love that, it narrows his options and increases their chances of stopping his progress.
Although still able (out wide) to get past one man, the defenders (like they do with Rashford) now come at him (albeit loosely outside the box) lined up one after another, he can beat one, or even two, but the third always gets him, happened quite a few times yesterday. He's seems to think he can only pass the ball into the box or across the goal when he has taken someone out, he just doesn't get to do that these days because whilst he retains his approach, most defenders have adapted theirs.
Arsenal have top defenders especially in the centre and whilst when he draws them out wide, he can trouble them, individually, but once in the box they spring their trap, and he's up a blind alley, off balance and unable to retain the close ball control because of the numbers directly in front of him.
He's becoming a 'copy cat' of Rashford out wide on the left, but he doesn't have the body strength for that 'bustling through' style. He needs to draw defenders out, getting them one on one when he can and, most significantly, chasing him from behind (or from the side) not in front of him.