not sure about your minigrids. like I said I have a huge area I've clearcut, and the only grass there is the grass I planted(maybe 1 or 2 have converted naturally but I haven't noticed)
my meaning here is that my house is on top of a nice large hill. I set my boundaries from the north side of the hill to the south and from a curve on the east side, with no natural boundaries on the west side(I'll add a yet here since I'm still growing). I have completely clearcut the east and the north side of my property, and have about 5 fir trees on the southern edge of my property(cone cows) I haven't cleared the west side yet because I don't have plans for it, and don't have the storage for the blocks/boards. my current enclosure is about 1/3 of the pink gridlined area in the minimap(my grid seems bigger than yours though)...if it went the way you're saying almost all of my land(except the paved parts) would have turned into grassland, but besides what I've planted none has.