I may be alone in this, but after encountering this confusion again after a couple of months of acclimation, I feel compelled to raise the problem Gmail has with overloading the "up and left" arrow.
Attached are two images capturing different uses of the shape. One is clearly labeled "Reply". The other is familiar only to Android users and people who remember what function was to the left of the Archive button before the themes changed.
Yes, the arrows are in two different places. Unfortunately, I spent too many years in other mail clients where the reply button was in the top left corner of the window. Before this button arrived, believe it or not, I'd sometimes click Archive, unconsciously expecting the button in that location to be Reply.
Yes, one arrow is rounded and the other is square. Years of recognizing characters in different fonts, and understanding stylized signage, seem to have trained me out of caring about the subtle features of the shape, though.
I've learned to love the keyboard shortcuts that Gmail provides, so I'm only rarely caught by this confusion. I do think it's funny in the context of this confusion, though, that the QWERTY keyboard places these shortcuts in the same finger position on opposite hands – 'r' and 'u'.