Quite simply, increasing your game resolution will make the incoming mob sheets appear smaller. While at first blush this makes perfect sense, the rest of the UI elements are correctly scaled up upon increasing the resolution which makes the difference rather stark.
On smaller, higher resolution screens (or in situations where display scaling is implemented, such as VSR, DSR or similar) this leads to the sheets becoming difficult to read.Expected Results: All UI elements would scale with your chosen resolution - not just those without Z values.
Even when scaling them up they're still bizarrely small at higher resolutions. This does, as you probably summised, only occurred once past 1920x1080 - such as, for example, 4x DSR to 3840x2160.
Seen as the elements appear to be able to scale down to a lower resolution (with mixed results), surely they could also be configured to scale up to higher ones.
Have you tried using your cursor to focus one of the enemy billboards that both scales them up so you can see them better and make them more opaque? Also what resolution are you looking at this on. 1920x1080 tends to be the standard max resolution for most players and appears to display enemy billboards in a readable scale.