Steps to repro:

1. Open Unity;
2. Create new project;
3. Import attached 'terrainTextureBlending.unitypackage' package;

4. Notice that there is a terrain with several blended textures painted (5 textures with different opacity and target strength);
5. Scroll out in a scene view;

6. Notice that some textures disappear at certain viewpoint. Further scrolling out brings them back;
7. Select 'Main Camera' gameobject in Hierarchy view;
8. Increase Y position by clicking on Y in the Inspector position field, holding left mouse button and dragging it;

9. Observe the same behaviour in the Game view.

Expected result:
Blended textures should not disappear when changing viewpoint.

Notes:
- Reproducible in: 5.4.0b6, 5.4.0b5, 5.3.2f1, 5.3.1f1;
- Reproducible in OSX 10.11.3;
- Game view texture disappearing doesn't reproduce on Retina screen (only on external monitor). Scene view disappearing reproduces on both Retina screen and external monitor;
- The issue happens only when the Scene view is scaled and doesn't happen in the fullscreen Scene view;

- Please note, that the bug doesn't reproduce always. For increasing reproduction possibility make sure that you did following:

1) Added 5 or ore textures to Paint Texture tab of the Terrain component;
2) Painted at least 3 textures overlapping each other with different opacity and target strength;
3) Have changed heightmap;
4) Have at least 3 textures overlap (one texture on top of each other), the highest one has less opacity and target strength. Also the highest-layered texture should be painted on a hill and on surface at the same time;
5) Have realtime auto lightmapping turned on.

If I find how to reproduce this bug more easily, I'll update description steps as well :)