Mesh triangles are clipped before tessellation

Background: I'm using a surface shader with tessellation turned on in order to increase the resolution of a mesh and displace it with a heightmap. This pretty much works great except in one situation...

Problem: It appears that the triangles of my mesh are being clipped before they are tesselated and displaced. So if the un-tessellated and un-displaced triangle is not within the view frustum, it is not rendered, even if it would be visible after tessellation and displacement.

Example: Two identical meshes, one with the tessellation shader and one with a standard Diffuse shader (no tessellation/displacement). If the "base" mesh is completely visible, everything works great:

However, if some triangle are not in the view frustum on the "base" mesh. They aren't displayed, even though they will be visible after tessellation and displacement.

Has anyone encountered this before? Have I done something wrong in my tessellation shader? I would expect clipping to occur after the tessellation and vertex fragments.