If you turn on “Double-sided” with a material whose surface is inverted and invisible, it will turn black.
(The door part is mainly inverted.)
If you look at a similar post, I think it’s because you can see the back side that is not exposed to light.
At present, I think it is necessary to correct it with CAD software,
It would be great if we could add features that could be fixed in Shapespark.
The CAD software maker has asked me to modify the FBX file in the destination application.
Thanks for the suggestion @Eri, but I am afraid we won’t be able provide such a function. Shapespark focuses on visualization of 3D models and leaves the modeling aspect to other tools (Shapespark assumes that all the operations which modify geometry are performed in the 3D modelling tool the model is created in).
Does the issue occur for any kind of door added in your CAD software? Or only to a certain kind? Are you using some 3D modeling tools (eg. Blender) along with the CAD software? If so, you could import the FBX file to the modeling tool, fix the reversed face issue there, and export the fixed FBX file from the tool.