In the game there are a few operations that can turn grass blocks into dirt.
Such operations are:

• tree growing (a block under the sapling is replaced with dirt)
• sheep eating grass

From what I've observed on my survival world, in both of these cases grass turns into vanilla dirt block. While I don't know if that's intended behavior, I feel like it would be QoL improvement if in both of those cases the grass turned into mod's equivalent of dirt - Artificial Soil which would allow the nearby grass to regrow to that block.