The observed issue is caused by incorrect mesh bounds being generated in your MakeMesh method in TerrainTussock.cs.

The generated grass mesh is being assigned a bounding box of new Bounds(-Vector3.one* 50000, Vector3.one * 100000)
which means a bounding box centered on (-50000, -50000, -50000) of size (10000, 10000, 10000)
The vertices of the generated grass mesh are placed outside this bounding box, with positive coordinates.

When DrawMeshInstanced is called, if the camera is facing away from the negative axes, the incorrect bounding boxes of the grass meshes are causing them to be culled, resulting in the observed issue.

One solution would be to generate an accurate bounding box for your mesh when generating, from the actual vertices you are generating.

Alternatively, if you definitely want a very large bounding box then it should have the center positioned such that the bounds will actually cover the vertices of the mesh.
For example, using
mesh.bounds = new Bounds(Vector3.zero, Vector3.one * 100000);
instead of
mesh.bounds = new Bounds(-Vector3.one* 50000, Vector3.one * 100000);