That it works at all in the Auctiva editor is surprising. It just depends on what the text and control characters are in Word. Some are OK in the editor but many are not. Knowing this, we've always advised people not to create descriptions in Word and paste them into the editor. There have been numerous posts about that in the forums over the past 2-3 years.
Re the eBay editor, that is proprietary code to them, so we don't know how it works. However, I would bet that it will have problems with MS-Word created descriptions too under some circumstances.