Just FYI for the people googling this in the future: I had this exact same problem with a CP201x usb-serial adapter. The solution is to change all the "ATTR" references to "ATTRS" in the udev rules. That's it. That's the magic incantation. I hoped this helped someone.
In this case changing ATTR{manufacturer}=="Prolific Technology Inc." to ATTRS{manufacturer}=="Prolific Technology Inc." most likely would have fixed OP's issue.