Furnances will not currently save their NBT Data correctly.

Reproduction:

1 - Place a furnace, insert coal / wood and wait for an item or two to cook.

2 - Observe the RecipesUsed slots increasing as expected.

3 - Save and exit your game; and try loading the world again.

4- Furnace NBT tags throw a malformed error, and are now empty; losing all items inside, with a new Blank Block Entity.

[BEFORE]

/setblock -141 63 67 minecraft:furnace[facing=north,lit=true]{BurnTime:1340s,CookTime:61s,CookTimeTotal:200s,Items:[

{Slot:0b,count:63,id:"minecraft:oak_log"}

,{Slot:1b,count:63,id:"minecraft:coal"},{Slot:2b,count:1,id:"minecraft:charcoal"}],RecipesUsed:{"ResourceKey[minecraft:recipe / minecraft:charcoal]":1}}

 

[AFTER]

/setblock -141 63 67 minecraft:furnace[facing=north,lit=true]{BurnTime:0s,CookTime:0s,CookTimeTotal:0s,Items:[],RecipesUsed:{}}

[RECOMMENDED ACTION]

• Create a proper NBT CODEC method to parse the Recipe Entry / Resource Key properly, with the Integer Values as a separate NBT key under the Compound Tag.