I tried figuring out which slot it considered the input slot to not let it do that, but it stopped pulling altogether , so for furnaces it seems like the only options are to extract all, or extract nothing.

Are you aware that the second slot you type in is exclusive? Meaning that if you type in slots 2-3, it will only pull from slot 2?
If it still doesn't work then, this is a bug.

yes, I am aware of this.
still , it wasnt working.
i'll have another look if you like

Edit: and while useful in some edge cases,( like extracting buckets of water from the fuel slot automatically after drying a sponge) , it's still counter-intuitive that it can extract items from the fuel slot, when a hopper cant do it.

tested again, it's still all or nothing

Furnaces are stupid.