Bug Why are the sides of an imported Blender mesh not showing up correctly?

I created a cylinder in Blender with a shape key to vary the thickness of the interior wall. It works and looks great in Blender, but for some reason is not rendering properly in Unity. Some faces are invisible from different perspectives, and you can see completely through the interior wall. Does anyone know why this is happening?

 

One word: normals.

More than one word: the normal vector of these faces is flipped. In 3D software there's something called "backface culling": faces that are not looking towards the camera are not rendered, to save resources. Blender has backface culling disabled by default, and Unity has it enabled by default.

Select those faces in Blender and flip their normals. Also, see:
https://artisticrender.com/what-is-backface-culling-and-when-is-it-useful-in-blender/