Ancient Debris drops only one netherite scrap when using a pickaxe with both smelting touch and fortune to dig it. Fortune works successfully with smelting touch when digging other ores (iron ores, gold ores, metal ores in other mods, etc.). These ores drop multipled ingots (smelted raw minerals) depend on the levels of Fortune enchanted on the tool.

That is because the smelting touch enchant is applied to the drops after fortune, and in vanilla, you can't fortune ancient debris, so this is intentional.