Normally I would have said that the back leg (for example) is overlapping the belly a little and you are seeing the line of the belly drop out at that point. But in that gap is another line (the hidden texture perhaps since its thinner?) see image.
S&T can be tricky and can produce popping line segments depending on geometry.
Can you upload your project and we'll have a look at your settings and the model to see if there is a way around it.