Well, that was unexpected. Nature is mocking me.
I channeled a big area on the surface (leaving out an area in the middle of it), just to ... watch the grass growing. And yes, it did grow. Then I dug the area I left out. No grass. The effin' grass needs sunlight now. What is this, I don't even?
It worked just fine just yesterday (and about every time I played DF, with or without 31.19.), so it must be related to the biome or the ocean.
Edit: Okay. There's a shrub and a tower-cap sapling growing in my Main Hall. The Main Hall is on a different biome than my tree nursery (which is still barren). There's no sign of grass regrowth anywhere else but in direct sunlight, though.
In my mountain/forest fort, I find I get fast growth of cavern vegetation in my dug out soil layers (i.e. at least one full layer of soil under the floor, even if I dig the soil out later to leave just a thin "soil floor"). Conversely, I cleared a big cavern space in the first cavern layer, hoping to get a tree farm up, but I find that grass/mushrooms don't grow in dug out gabbro, just on the "sandy loam" floors in between the old pillars.
Also my fort's main entrance levels (three layers of soil), gets lots of towercap/fungiwood, despite never being exposed to sunlight. I dug out a pasture room three soil levels down (again, no exposure to sunlight), and that gets lots of grass (feeding my alpaca herd quite nicely).
I think in fixing the "mud needed for underground farms" bug, it removed the ability for wild plantlife to spread across muddied stone (and increased the growth on non-muddied soil) even if right next to existing fungus/tower-cap squares.
does your exposed cavern layer have fresh water or salt water? (I think, but I am not sure, that you can check for fresh water by successfully creating a drinking zone over it).