Poser 12.0.471. Results are the same in Poser 11.

Red Phantom is correct! It's the skinning method that affects these renders, so I guess this isn't a bug.

I've got 3 Poser gals here and left skinning at their defaults (LF unimesh, Pauline unimesh, Jessi traditional). I conformed their shirts on them, left their skinning at defaults (same as their character figures), and scaled the clothes down overall to 95%. I also disconnected any displacements.

Then I went in to each of the character figures and unchecked Visible in Camera and Visible in Raytracing for each of their right collars and right shoulders. I unchecked Cast Shadows for these, too. I kept the Visible box checked for these.

Then I went into the clothes and unchecked Cast Shadows for their right collars and right shoulders.

Here's the preview (screen capture).

Firefly render.

Superfly render.

Superfly with all clothes traditional skinning.

Superfly with all clothes and all characters traditional skinning.

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W11,Intel i9-14900KF @ 3.20GHz, 64.0 GB RAM, 64-bit, GeForce GTX 4070 Ti SUPER, 16GB. 

Old lady hobbyist.

All visual art or fiction is "playing with dolls."