Well, the reason gravity breaks is because these things go over the speed of light.
Usually this is not possible, because as something speeds up, it get more and more massive, and therefore the acceleration becomes less and less, until the speed caps at the speed of light. If it surpasses that, the equation gives an imaginary number. Gravity and acceleration are dependent on mass. So those become imaginary as well.
But these don’t speed up to the point. They are already there.