I'm not completely sure but it looks like your nameplates are "clamped" to that position; the game doesn't want to move them any further up, so it just starts overlapping them instead. There are options for clamping in /knp > cvars; you might want to reduce the top clamp distance. The game itself controls nameplate movement. Other than the stacking/overlapping option and a few other cvars (which means "console variable"; they're settings built into the game), add-ons can't move them because they're restricted frames. So if this isn't because they're clamped, there's nothing else you can do. It's something blizzard would need to change.