Farmland and dirt path don't convert into dirt when water or lava is above them

The Bug:

Farmland and dirt path don't convert into dirt when water or lava is above them.

Steps to Reproduce:

1. Place some farmland or dirt path down.
2. Place some water above the farmland or dirt path.
3. Take note as to whether or not farmland and dirt path convert into dirt when water, lava or powder snow is above them.

Observed Behavior:

Farmland and dirt path don't convert into dirt.

Expected Behavior:

Farmland and dirt path would convert into dirt.