Giving the following as input:
SurfaceAndGroundControl: minecraft:air, minecraft:dirt, minecraft:dirt, -4.2, byg:peat, minecraft:coarse_dirt, minecraft:dirt, -3.9, minecraft:grass_block, minecraft:dirt, minecraft:dirt, 3.9, byg:peat, minecraft:coarse_dirt, minecraft:dirt, 4.2, minecraft:air, minecraft:dirt, minecraft:dirt, 7.0
gets written back as this:
SurfaceAndGroundControl: minecraft:air, minecraft:dirt, minecraft:coarse_dirt, -4.2, byg:peat, minecraft:dirt, minecraft:coarse_dirt, -3.9, minecraft:grass_block, minecraft:dirt, minecraft:dirt, 3.9, byg:peat, minecraft:dirt, minecraft:coarse_dirt, 4.2, minecraft:air, minecraft:dirt, minecraft:coarse_dirt, 7.0

I had a peek at MultipleLayersSurfaceGenerator, but I can't pin down the cause. Something definitely seems to be strange about this though, with how some dirt become coarse_dirt, and some coarse_dirt just disappear. Should test and see if the same configuration with different blocks gives the same result, or if this is unique to coarse_dirt

Tried to reproduce with:
SurfaceAndGroundControl: minecraft:air, minecraft:dirt, minecraft:dirt, -4.2, minecraft:gravel, minecraft:coarse_dirt, minecraft:dirt, -3.9, minecraft:grass_block, minecraft:dirt, minecraft:dirt, 3.9, minecraft:gravel, minecraft:coarse_dirt, minecraft:dirt, 4.2, minecraft:air, minecraft:dirt, minecraft:dirt, 7.0
but it remained after writing. This issue is either already fixed, depends on interactions with modded blocks, or was misidentified