Every frontend dev I've ever worked with has received this same treatment, so I would find it difficult to believe it's even predominantly females; but maybe I've met a lot of exceptions.
But, I am not even frontend developer and get the assumption that I am one. When I was nearby frontend, people made further assumptions about which part of it I do - they expected me to do aesthetic work altrough I am really bad at it and were sometimes oddly awkward when I wanted to talk about architecture or algorithms. I don't think male frontend developers are not undervalued, but the assumption about which part of it they do is on average a bit different.