I don't know how else to describe it, so here is a screenshot to demonstrate what I'm talking about.

To reproduce:

1. Be wearing an item whose appearance has 3D bits (e.g. the sleeves on this chestpiece)
2. Open dressup
3. Choose a race to preview (can be you own)
4. Undress all

This is known error and limitation due to limited control of dress up model frames. To fix this Blizzard needs to do it, unfortunately there's no way I can help it.

Aaah, gotcha! Fingers crossed they get around to addressing it then. Thanks for clearing this up.