It seems that datapack %world%/datapacks/loot_tables/data/minecraft/recipe/gold_ingot.json

(same for others smelting recipes)

Does not allow to give more than "1.0" experience.

AND I know that the actual code of furnace does not allow to output more than 1 item and need to rewrite the furnace code, but can you please change that and allow datapack to modify the amount of output of a smelting. thanks