Notice how the battlefield scene is very wet. That makes the basecolors dark and adds specular to everything which means when looking down you will still get nice glints of the sky from the wet specular.
Losing normal map detail for overcast settings is a pretty common problem and generally means you need to pay more attention to having nice cavity type details, or making sure the HDR sky has at least a nice gradient going on. Often times people end up using overcast HDRs that are very symmetrically even and those can be a challenge to maintain detail.
This is why you will often see things like lots of variation in wetness, or maybe moss that is vertically aligned which is just another way of hinting at the tiny normal map details. Or like the Char level from gears2, a light layer of dust on everything can also be effective. But even so, having nice variation in your roughness maps becomes much more important than normal maps for many of these kinds of scenes.