Grass under water only decays during day time

Steps to reproduce:

1. Place water on the ground;
2. Set RandomTickSpeed to 4096 to make it easier to notice;
3. The grass will decay;
4. Set time to midnight.

Expected results:
The grass stays dead, and other water blocks placed above grass during night time should also decay the grass underneath.

Observed results:
The grass grows again during night time.