I've reverted this, for the same reason as https://www.openstreetmap.org/changeset/151555979 .  "farmland=grassland" is an unusual value, but it's surely less incorrect that "natural=grassland", which isn't farmland at all.
If you're familiar with the area, perhaps have a look at https://taginfo.openstreetmap.org/keys/farmland#values and see what specific value (not counting generic stuff like "field") best describes the nature of the farmland here?