Not sure that was the case. From the one angle, you see him lose the ball but you can't see the body contact from the other side. From the back, you clearly see the contact, but you can't see when he lost the ball.
Then, even if what you say was the case, it's a loose ball foul for the body check.
There were three challenges tonight, IIRC, and they all were successful. The Mavs only took one that I remember. If they thought there was a chance of overturning it, they would have taken it.