Howdy!

I'm new to the Reallussion ecosystem. I've been trying out CC4 and I noticed something really awful in the models.
I was working on a plus sized character and I noticed there's this strange deformation at the navel. So I sent it to Blender and
FIXED the weights so it would deform better. BUT! to my surprise THERE IT IS AGAIN! It reappeared in CC4. Why is this happening?
It's just so awful whenever the character sits or bends forward a bit.

I even purchased one of the Stylized heaven models and THERE IT IS AGAIN! Why? It's sticks out like a sore thumb. Also
CC4 SHOULD LET YOU ADJUST THE FREAKING WEIGHTS!

Can anyone tell me how to fix this? It's just UGLY, please send help!