This is still the case in 5.3. Just to be clear, it’s just when you zoom in the editor. It makes working with 9 sliced textures a bit cumbersome, as it’s not WYSIWYG. It’s not reflecting the size on the canvas, rather it’s showing it as it’s rendered.
The fix would be to render it in 1:1 as it does now, THEN scale the result based on your zoom.