Is that ball special in any way or is a default Unity game object? Is there only one collider on that ball? Is the ball collider shifted in xyz? Looks like the C.G. is moved. Also, try to put a texture to the shader so that you can look at the revolution speed. You can submit a bug report so that Unity can track the issue. Include here the number
Nice to know that is working well. I'm curious: Cosa cerchi di far? Sembra una interazione interesante! raconta un po, in inglese.