I think the problem is caused by a new feature of P6--Smoothing Groups.

The P6 manual discusses these. They're not required, but if they exist P6 respects them.

The geometry of the Jessi Pink Bra has embedded smoothing groups. If I create a prop at the offending areas and export as an obj, the obj keeps the smoothing groups. P5 seems to ignore them and renders without artifacts. If I then export from P5 as an obj, there are no smoothing groups. So, re-importing the P5-created obj into P6 renders OK because there are no smoothing groups.

Also, after editing out the smoothing groups from an obj exported from P6 leaving everything else the same, P6 renders without artifacts. The smoothing groups of the pink bra cause artifacts at the odd triangulated facets, AND at the interface between body parts. I don't think (but I really don't know enough) that smoothing groups can span body parts.

We need to learn more about the proper application of smoothing groups.