I never resolved anything for this track, credits go to
@slider666 who fixed that floating grass.
Iirc (don't have it any more) there was no grassFX in the original and no issues with the floating grass either. That got possibly introduced later on by a config.
The 'weird grass' is just how it used to be before grassFX appeared. It does indeed look strange seen from above, but while driving it doesn't bother me at all.
So until someone sees a way to fix it, it will either have 'weird grass' or 'floating grass'...
EDIT: I just installed the Legion track again, got the floating grass but also the 'weird grass', supplemented with grassFX, so it's less obvious.