Mushrooms cannot spawn on marsh turf.

In terrain.lua:

red_mushroom = 		{WORLD_TILES.ROAD, WORLD_TILES.WOODFLOOR, WORLD_TILES.SCALE, WORLD_TILES.CARPET, WORLD_TILES.CHECKER, WORLD_TILES.ROCKY, WORLD_TILES.MARSH, WORLD_TILES.DIRT, WORLD_TILES.DESERT_DIRT, WORLD_TILES.MONKEY_DOCK },
		green_mushroom = 	{WORLD_TILES.ROAD, WORLD_TILES.WOODFLOOR, WORLD_TILES.SCALE, WORLD_TILES.CARPET, WORLD_TILES.CHECKER, WORLD_TILES.ROCKY, WORLD_TILES.MARSH, WORLD_TILES.DIRT, WORLD_TILES.DESERT_DIRT, WORLD_TILES.MONKEY_DOCK },
		blue_mushroom = 	{WORLD_TILES.ROAD, WORLD_TILES.WOODFLOOR, WORLD_TILES.SCALE, WORLD_TILES.CARPET, WORLD_TILES.CHECKER, WORLD_TILES.ROCKY, WORLD_TILES.MARSH, WORLD_TILES.DIRT, WORLD_TILES.DESERT_DIRT, WORLD_TILES.MONKEY_DOCK },
		

As you can see, mushrooms can't spawn on marsh turf, even though in terrain_marsh.lua, mushrooms are present in distributeprefabs:

AddRoom("BGMarsh", {
					colour={r=.6,g=.2,b=.8,a=.50},
					value = WORLD_TILES.MARSH,
					tags = {"ExitPiece", "Chester_Eyebone", "Astral_1", "CharlieStage_Spawner"},
					contents =  {
									countstaticlayouts={["MushroomRingMedium"] = function()
																				if math.random(0,1000) > 985 then
																					return 1
																				end
																				return 0
																			   end},
					                distributepercent = .25,
					                distributeprefabs=
					                {
										spiderden=0.003,
										sapling=0.0001,
										twiggytree = 0.0001,
										ground_twigs = 0.00003,
										pond_mos=0.005,
										reeds=0.005,
										tentacle=0.095,
										marsh_bush=0.05,
										marsh_tree=0.1,
					                    blue_mushroom = .01,
					                    mermhouse=0.004,
					                },
					            }
					})
	-- No trees, no rocks, very rare spiderden
AddRoom("Marsh", {
					colour={r=.45,g=.75,b=.45,a=.50},
					value = WORLD_TILES.MARSH,
					tags = {"ExitPiece", "Chester_Eyebone", "Astral_2"},
					contents =  {
									countstaticlayouts={["MushroomRingMedium"]=function()
																				if math.random(0,1000) > 985 then
																					return 1
																				end
																				return 0
																			   end},
					                distributepercent = .1,
					                distributeprefabs=
					                {
					                    evergreen = 1.0,
					                    tentacle = 3,
					                    pond_mos = 1,
					                    reeds =  4,--function () return 3 + math.random(4) end,
					                    --TODO: jcheng: fix mandrakes
					                    --mandrake=0.0001,
					                    spiderden=.01,
					                    blue_mushroom = 0.01,
					                    green_mushroom = 2.02,
					                },
					            }
					})
AddRoom("SpiderMarsh", {
					colour={r=.45,g=.75,b=.45,a=.50},
					value = WORLD_TILES.MARSH,
					tags = {"ExitPiece", "Chester_Eyebone", "Astral_1"},
					contents =  {
					                distributepercent = .1,
					                distributeprefabs=
					                {
					                    evergreen = 1.0,
					                    tentacle = 2,
					                    pond_mos = 0.1,
					                    blue_mushroom = 0.1,
					                    reeds =  4,--function () return 3 + math.random(4) end,
					                    --TODO: jcheng: fix mandrakes
					                    --mandrake=0.0001,
					                    spiderden=3.15,
					                },
									prefabdata = {
										spiderden = function() if math.random() < 0.2 then
																	return { growable={stage=2}}
																else
																	return { growable={stage=1}}
																end
															end,
									},
					            }
					})

Edit:

Similar issue with grass and desert biomes:

grass = 			{WORLD_TILES.ROAD, WORLD_TILES.WOODFLOOR, WORLD_TILES.SCALE, WORLD_TILES.CARPET, WORLD_TILES.CHECKER, WORLD_TILES.ROCKY, WORLD_TILES.DIRT, WORLD_TILES.DESERT_DIRT, WORLD_TILES.METEOR, WORLD_TILES.MONKEY_DOCK },

AddRoom("BGBadlands", {
					colour={r=0.3,g=0.2,b=0.1,a=0.3},
					value = WORLD_TILES.DIRT_NOISE,
					tags = {"CharlieStage_Spawner"},
					contents =  {
									distributepercent = 0.07,
									distributeprefabs =
									{
										marsh_bush = 0.05,
										marsh_tree = 0.2,
										rock_flintless = 1,
										--rock_ice = .5,
										grass = 0.1,
										grassgekko = 0.4,
										houndbone = 0.2,
										cactus = 0.2,
										tumbleweedspawner = .05,
									},
					            }
					})

AddRoom("Lightning", {
					colour={r=0.3,g=0.2,b=0.1,a=0.3},
					value = WORLD_TILES.DIRT_NOISE,
					contents =  {
									distributepercent = 0.05,
									distributeprefabs =
									{
										marsh_bush = .8,
										grass = .5,
										grassgekko = 0.4,
										--rock_ice = .5,
										lightninggoat = 1,
										cactus = .8,
										tumbleweedspawner = .1,
									},
								}
					})

and moles (and marshbushes) in MolesvilleRocky room:

molehill=			{WORLD_TILES.ROAD, WORLD_TILES.WOODFLOOR, WORLD_TILES.SCALE, WORLD_TILES.CARPET, WORLD_TILES.CHECKER, WORLD_TILES.ROCKY, WORLD_TILES.MARSH, WORLD_TILES.MONKEY_DOCK },

AddRoom("MolesvilleRocky", {
					colour={r=.55,g=.75,b=.75,a=.50},
					value = WORLD_TILES.ROCKY,
					contents =  {
									distributepercent = 0.1,
									distributeprefabs =
									{
										marsh_bush = 0.2,
										rock1 = 1,
										rock2 = 1,
										rock_ice = .3,
										rocks = .5,
										flint = .1,
										grass = 0.1,
										molehill = 1,
										grassgekko = 0.3,
									},
					            }
					})