Steps to Replicate:

* Convert "Underwear Bottom" to Accessory on Default CC project
* Export to FBX (I used Blender Target Preset with option to generate FBXKey)
* Use "Create Cloth, Hair, Acc (with FBXKey)" and import it right back
* Select "Underwear Bottom" and try to change something in UV Settings under Materials tab
None of the UV Settings controls do anything as oppose to UV controls for Bra material (which is a cloth)