Grass spreads under tilled dirt / farmland

Grass will spread to dirt blocks that are below tilled dirt blocks / farmland, as if there are no blocks on top of them. Perhaps the game is treating tilled dirt as a transparent block?
It is an issue because I want to keep grass out of my farms, so animals don't spawn in them. The designs that have worked since 2012 are far more difficult to fix because of this issue.
eg, for a pumpkin or melon farm, when you harvest the item it leaves behind a dirt block, that grass will quickly appear on. I found that grass was growing all under neath the farm, and upon removing it and replacing with dirt, saw the grass spreading to blocks under farmland.
The only solution was to dig out completely under the farmland to exterminate all the grass, which is a nuisance.