To reproduce:
1. Open attached Unity project "medzej.zip" attached by QA in comments section
2. Open scene "_Tree_Shadows"
3. Go into Play Mode

Expected result: Tree C's shadow should react to wind, move (Attached "Expected.gif")
Actual result: Tree C's shadow is static, does not move (Attached "Actual.gif")

Notes:
- Regression first introduced in: 5.6.0a2
- Reproduced in: 5.6.0a2, 5.6.0a3, 5.6.0f3, 5.6.1p1, 2017.1.0b5
- Not reproduced in: 5.6.0a1, 5.5.3p3
- Trees B and C are identical, the only difference is that all of Tree C's children have "Cast shadows" set to "Shadows Only"