If you are noticing the spaces between the pixels, it probably means that the monitor is too large for the viewing distance. The 1440 vertical resolution is associated with a range of horizontal resolutions, from 1920 to 5120. If the monitor is at the upper end of that range, it would be almost 200 dpi, and you probably wouldn't notice. If it's at the low end of the range, it could be under 90 dpi, which is a little coarser than a typical monitor.
But if you are noticing it, you either have the eyesight of an eagle or the monitor was designed to be viewed from farther away than the normal distance for a computer screen. A 27" monitor is pretty large for close viewing unless that space is filled with high density pixels.
As to why you can see it on the new one and not the old one, if the screens are the same size, have the same resolution, and you are viewing them from the same distance, are the monitors' dynamic ranges different? High dynamic range monitors often have a darker background to help differentiate dark colors. Even if it doesn't have high dynamic range, a darker background gives it more headroom to improve the definition, so you can find it on better quality monitors. If you didn't notice it on the old monitor, it could be that the size was too big for the viewing distance, but the effects of that weren't as noticeable because the background wasn't as dark.