Steps to repro:

1. Open Unity;
2. Create new project;
3. Import attached 'GrassTextureBug' package;
4. Open 'test' scene;

4. Notice that there is terrain gameobject with modified heightmap (mountains), heavily populated with billboarded grass (with maximum opacity and target strength level);
5. In the Scene View scroll closer to the grass hill, so that the viewpoint's position is upper relative to grass texture;
6. Move the viewpoint below by holding right mouse button and moving mouse cursor up.

Actual result:
Some chunks of grass texture (which is mainly in the center of Scene view) is not rendered. At some point, when the viewpoint is almost parallel to the ground grass texture gets again fully rendered. See attached gif video and actual.png screenshot. Please notice that this behaviour is inconsistent with rendering billboarded grass on even surface - in this case after toggling viewpoint angle grass rendering doesn't disappear.

Expected result:
Billboard painted-on-hill grass texture rendering should not disappear when viewpoint is above it (see attached expected.png screenshot).

Workaround is to turn off Billboard checkbox in Edit Grass Texture window for current texture.

Notes:
- Reproducible in: 5.4.0b7, 5.4.0b6, 5.3.2p4, 5.3.2f1, 5.3.1f1, 5.2.2p4.
- Reproducible on both Windows 10 and OSX 10.11.3;
- Not reproducible when grass billboard is painted on even surface;
- Not reproducible with billboard mode for grass texture is turned off;
- Also reproducible in Game view (see attached video and actual.png screenshot).