Well, borderless printing is ultimately a hack. From mechanical inaccuracies in the printer to the printer driver adding further scaling of the image there's a million things at play. This by all means mustn't be PS' fault at all. It's a moot point, anyway. You have not offered up anything precise like system info, exact printer settings and exact version of PS, so we can't even begin to tell you which knobs to tweak. that said, you likely will still have to figure it out on your own. What usually works is to leave everything default in PS (or for that matter in other apps) and tweak the margins in the printer's preview app. that's of course assuming the printer was mechanically calibrated correctly in the first place. to save ink and paper I also would use a simple grid image. This helps to figure out which internal offsets the printer uses and you can easily measure it out based on teh grid spacing.
Did you ever solve this issue? I used to be able to print perfect borderless on 8.5x11 on my P600, but ever since the 9.86 printer driver came out with the "Max/Min" settings I haven't been able to print true borderless.
I‘m using PS cc 2018 and Canon prograf 1000 with PS print module. Under paper size with Canon,there are several paper sizes that come borderless. If the borderless sizes not listed, I have to make my own custom sizes to be borderless,such as 11x14,etc.I don’t have experience with Epson so don’t know how much of a help I am.