Why doesn't rigid body physics work properly after I add the Build modifier? I have a cube, a plane and a ball. 

At first, I set the rigid body physics of the cube and the plane to passive and the ball to active. I set the strength of the force field physics of the cube to 0 at frame 1 and at frame 50, when the ball falls on the plane, I set it to -100. As expected, the ball falls on the plane and the cube attracts it. 

But, after that I use the loop cut tool to cut the faces into multiple faces. 

I add the Build modifier and set its start frame at 1 and length to 50. 

But now, the ball doesn't hit the cube, but rather goes through it and falls into the bottom. Why does that happen?
I suggest to add an invisible wall to bypass the problem of the cube physics when the build modifier is attached to it. So the ball will collide and bounce on that wall.

This is a quick experiment where the cube acts the way you need (I don't know why it works):

https://filebin.net/zisptqbp3v67qm9c