When using an oversized lava bucket as fuel (alternatively, I believe some item with a recipe remainder added as a furnace fuel through a tag would work), then the recipe remainder is lost upon the fuel's consumption, rather than simply replacing the fuel slot as would be possible with a stack of size 1. I would expect the bucket that is produced to remain in some way, even if it involves dropping in the world as brewing stands may do.

Steps to reproduce

1. Obtain an oversized lava bucket stack: /give @s minecraft:lava_bucket[minecraft:max_stack_size=99] 99
2. Place the oversized stack into the fuel slot of a furnace.
3. Start smelting with some other item in the other slot of the furnace.
4. Observe that the fuel slot now contains 98 lava buckets, rather than 98 lava buckets and an empty bucket.