Grass spreads to tiles with property NoSpawn Grass

[Sicariana]

I have made a custom farm with some tiles that contain the property 'NoSpawn Grass'. I do not want any grass to spread to these tiles, but grass still spreads onto these tiles. I'm not sure if this is a bug or something I've messed up in my custom map. Thanks.

[bcmpinc]

That property only affects spawning of new grass not its spread.