FYI, this has been reported in Mac forums independant of any Adobe software. Seems to be another famous Mac "feature" that comes out of nowhere for no purpose. I know this forum is full of "it's not us, it's them" responses but this one is a macOS Sierra deal. Many posts about this the first day Sierra was released, maybe the 10.12.1 update will address it.
I've not been able to reproduce this issue. There are some spots where I click in the dialog in both Sierra and El Capitan (in between files or on the file extension) where it doesn't select the name. But if I click on the file name or thumbnail it always changes the file name for me.
macOS 10.12.1 came out today. Wouldn't hurt to install that and see if corrects your issue. Otherwise, I video of your steps may give us a clue as to what you're doing differently from the steps we're taking.