This is a common problem of all ground shadows in all rendering packages. A really useful solution would be having a viewport-dependent safe zone so that no pixels get drawn in this buffer around the viewpoint edges. Basically like a blurred erasing airbrush that goes around the view and makes sure it fades all viewport boundaries to pure transparency.
I could code that in 1 hour if I knew how to code. But because I don’t, I can happily say it only takes 1 hour 