Hello ,
Thank you for posting in the community your questions and we would need to investigate further why this is happening would you be able to contact us and report a bug by following this link: https://twinmotionhelp.epicgames.com/s/case-community-page
Please provide the udatasmith file and the assets folder that gets saved with it so we start looking at the issue. If you can also provide a Vectorworks file that would be helpful as well so we can trouble-shoot the direct-link.
Kind regards,
Vincent B.