Hmmm, that's very interesting.
I did a quick test with clothplane at its default size along with grounp prop visible and shadow rendered poorly on both. I just did another render with now ground prop scaled down to 20% from 100% and, guess what, the shadow rendered perfectly!

I am curious, how those shadowcam determine this "zoom out"? The ground prop is originally imported from obj file and, even though polygons are huge, it is in default scale of 100%. Also, how those this zooming out effect shadows on other objects? In another word, if I mess around with the values in shadowcam to adjust the shadow on the ground, would it also effect other objects?

Thanks for the tip btw.