strange black square when casting raytracing shadow on folliage

I have a troublesome problem when the light casts raytracing shadows to the folliage. If the size of the object is small, everything is normal, if i scale the size, black spots will appear

The default size of the ball is fine

When scaled the size in to 3, such dark square appear

If it is not a folliage object, it will not have such a problem even if it is large. I found the same for 4.24, with the same results on 2080 or 1070
Is there someone who can help to solve
thank you

hope someone to help