Reproduction steps(GIF attached):

1. Open the attached project.
2. Go to "Assets > Handpainted Forest Environment Free Sample > Terrain Trees > Prefab" folder in project view.
3. Select "tree_01_t" prefab.
4. Click the prefab preview window.
- Lighting angle changes.
5. Select "tree_02_t" prefab.
6. Click the prefab preview window.
- Lighting angle changes.

Actual behavior:
Inconsistent lighting

Expected behavior:
Consistent lighting

Note:
Reproduces 95% of the time ( sometimes, lighting angle changes without a click ).

May be related:
case 860153

Reproduced in:
5.4.4p3, 5.5.1p4, 5.6.0b9

Resolution:
Preview window is not updated every frame for performance reasons.