No, I don’t think you’re doing anything wrong, since you can see a difference between 300m distance field shadow distance in the first image and 1000m in the second image (also means your trees are further away than 300m).
So now they do cast shadows also at a distance, but imposters tend to look flat (unless you use something like this here Post Page). You could decrease the strength of the sub surface scattering inside the imposter material, that will make them darker against the sun.