When previewing a recipe (without the required materials) in a furnace while its being fed by a hopper, items will be fed into the furnace which overlaps with the recipe preview. This also causes overlapping text.
Expected behaviour:
It'd be expected that the recipe preview should disappear when the furnace is being fed items.
Actual behaviour:
The recipe preview remains under the items entering the furnace.
Steps to reproduce:
Place a furnace with a hopper feeding into it and fill the hopper with cobblestone.
Selecting a recipe in the recipe book (make sure you don't have the materials within the recipe)
Observe the furnace being filled up while the recipe preview remains.
As you don't have any netherrack in your inventory, the recipe book tries to show a netherrack preview in the top slot of the furnace. To do that, it'll move all items in that slot into your inventory. Then the hopper fills up that top slot again. To me, this looks like intentional behaviour.