Grass fades under upside-down stairs (except oak wood ones)

Description:

Grass blocks turn into dirt blocks after a while under upside-down stairs. The only exception is the oak wood ones (grass stays grass under them).
As you can see on the following screenshot it's pretty ugly.

To reproduce:

Just place upside-down stairs on grass and wait.

How I think it should be:

Grass should not fade under any type of upside-down stairs.