So I tried to add the furnace from Misty World into the config file but I cannot get the syntax right. The furnace uses a very complex fuel system instead of a simple block state. I guess I'm asking to add support to mist:furnace. Here is the mod version as well as full block nbt I got from tellme.

SimpleDifficulty-1.12.2-0.3.9

Furnace (mist:furnace - 1377:12) has a TileEntity
Full block state: mist:furnace[facing=south,status=3]
Hardness: 3.5000, Explosion resistance: 3.5000, Material: ROCK
Block class: ru.liahim.mist.block.gizmos.MistFurnace
IBlockState properties, including getActualState():
PropertyDirection{name=facing, clazz=class net.minecraft.util.EnumFacing, values=[north, south, west, east]}: south
PropertyInteger{name=status, clazz=class java.lang.Integer, values=[0, 1, 2, 3, 4]}: 3
TileEntity class: ru.liahim.mist.tileentity.TileEntityMistFurnace

TileEntity NBT (from TileEntity#writeToNBT()):
TAG_Compound (10) ('')
{
TAG_String (8) ('id'): "mist:furnace"
TAG_Int (3) ('x'): 45169
TAG_Int (3) ('y'): 23
TAG_Int (3) ('z'): 27504
TAG_String (8) ('Lock'): ""
TAG_Int (3) ('BurnTime_0'): 363
TAG_Int (3) ('ItemBurnTime_0'): 1600
TAG_Float (5) ('CookTime_0'): 0.0
TAG_Int (3) ('CookTimeTotal_0'): 200
TAG_Int (3) ('AshProgress_0'): 437
TAG_Byte (1) ('Close'): 0
TAG_Byte (1) ('Signal'): 0
TAG_Int (3) ('BurnTime_1'): 363
TAG_Int (3) ('ItemBurnTime_1'): 1600
TAG_Float (5) ('CookTime_1'): 0.0
TAG_Int (3) ('CookTimeTotal_1'): 200
TAG_Int (3) ('AshProgress_1'): 437
TAG_Float (5) ('Temperature_0'): 2699.0
TAG_Float (5) ('Temperature_1'): 2849.0
TAG_Float (5) ('Temperature_2'): 2949.0
TAG_Float (5) ('Temperature_3'): 2999.0
TAG_Float (5) ('Temperature_4'): 2999.0
TAG_Float (5) ('Temperature_5'): 2999.0
TAG_Float (5) ('Temperature_6'): 2999.0
TAG_Float (5) ('Temperature_7'): 2999.0
TAG_Float (5) ('Temperature_8'): 2949.0
TAG_Float (5) ('Temperature_9'): 2849.0
TAG_Float (5) ('Temperature_10'): 2699.0
TAG_List (9) ('Items') (2 entries of TAG_Compound (10))
{
TAG_Compound (10) ('')
{
TAG_Byte (1) ('Slot'): 2
TAG_String (8) ('id'): "mist:ash"
TAG_Byte (1) ('Count'): 1
TAG_Short (2) ('Damage'): 0
}
TAG_Compound (10) ('')
{
TAG_Byte (1) ('Slot'): 3
TAG_String (8) ('id'): "mist:ash"
TAG_Byte (1) ('Count'): 1
TAG_Short (2) ('Damage'): 0
}
}
}

I don't know what to do at this point, I tried to incorporate "status=3" into the config file but no matter what I couldn't get it to work. If anyone can help, please do.

FALSE ALARM - I solved it. Turns out after reloading the file in-game it rearranges the entries so I was trying to add the same entry twice. Here is the working syntax.
"mist:furnace": [
{
"properties": {
"status": "4"
},
"temperature": 2.0
},
{
"properties": {
"status": "3"
},
"temperature": 4.0
},
{
"properties": {
"status": "2"
},
"temperature": 3.0
},
{
"properties": {
"status": "1"
},
"temperature": 0.0
},
{
"properties": {
"status": "0"
},
"temperature": 0.0
}
]