When scrolling with either up/down or left/right, if you just use an action it'll snap back to your previous target after. If you hit the confirm better (Should be X on PS4), that will become your new main target that it will snap back to. So, to reiterate, press confirm to lock in a selection, and if you don't do that you'll just be targeting it temporarily (Very handy for throwing out a heal on a DPS).