I encountered a strange disappearing part in a build I was working on today and thought I'd pass along my solution to it.
If you see parts of an object disappearing at different camera angles check your mesh in your 3D program for unused materials that seem to linger and get embedded into the FBX file. This can happen if you join some parts together then later or ever remove parts from a combined object. I fought with this a while and finally what fixed it was removing them lingering unused materials. This was in blender, so I don't know if it applies to other 3D modeling programs or not.
These extra unused materials may not even show up in the materials upload section, I didn't look to see if they did.
Thanks to Damius in support for pushing me to look deeper