Either these faces are missing or if they are just visible from one side, then the Normals are inverted. If you are using blender to make your Assets: Go in edit mode and press ctrl+N to let blender calculate the normals again or press space and search for flip normals. Flip the normals on the affected faces. There also is a Check box to render the backface Culling in Blender in the the right panel of the viewport (press N to show the panel), the checkbox is under the ‘Shading’ tab.
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