Hello ,
Specifically for geometry that appears as transparent when view from one direction, this is because the materials applied to the geometry were imported as one-sided materials, which is the case when importing from certain programs. If you select the materials using the Material Picker, you can enabled Two-sidedness for the materials to fix this.
For the missing geometry, first check enabling two-sidedness for the materials fixes this issue. If the geometry is still missing, please submit a bug report here and share your Twinmotion file and imported model for our support team to investigate this further
Best,
Raghib