Modifying the RecipesUsedSize tag from a furnace results in the creation of the tags RecipeLocation0 and RecipeAmount0 with the values of respectively "minecraft:" and 0.
The first value is obviously wrong, and the latter value is just weird.

The game just seems to ignore and remove that entry when depositing the XP to the user, but it stays as long as the user does not take out any item.

Steps to recreate:

1. Place a furnace

2. /data get block <x y z>

3. Everything should look just fine

4. /data modify block <x y z> RecipesUsedSize set value 10s

5. /data get block <x y z>

6. Now the weird values are present. Note that, even though the value 10s was given, only one recipe usage entry was created.
   a. RecipeLocation0: "minecraft:"
   b. RecipeAmount0: 0
   c. RecipesUsedSize: 1s

Note: in the example I've used the value 10s, but any positive non-zero short works.