Hello ,
Thank you for posting in the community and we would need to investigate further the issues with the FBX import and how this could be happening. Here is the link to report a bug: https://twinmotionhelp.epicgames.com/s/case-community-page
You can also try the Datasmith exporter plugin with Revit as this has been worked on to improve the workflows with Revit and Twinmotion.
Datasmith Exporter Plugin:
https://www.twinmotion.com/en-US/plugins/revit
Datasmith Exporter overview article:
https://twinmotionhelp.epicgames.com/s/article/Getting-Started-using-Datasmith-Workflows-with-Twinmotion
Datasmith Exporter overview video:
https://www.youtube.com/watch?v=UBne5zZ9S8o
Hope this helps.
Kind regards,
Vincent B.