Unity terrain and odd frustum culling behavior

I noticed this odd behavior today where the camera is facing away from the terrain yet the terrain is still being rendered as you can tell by the vertex count in the stats area of screenshot #1 (293 verts being rendered). However, if I rotate the camera slightly, it then doesn't render the terrain, see screenshot #2 (only 4 verts being rendered).

Does anyone have an explanation for this? I have literally turned off every possible thing I can think of (lights, fog, skybox) and I can't fathom why this is even happening. It has to be the terrain because if I delete the terrain object, the verts go back to 4. My bet right now is that it's a bug...

Open the images in new tabs to compare the stats better.

Screenshot #1 - 293 verts:

Screenshot #2 - 4 verts: