The way the game pulls from inventories to craft can be tricky, but in the case of an item being useable as more than one ingredient in a recipe compounds the trickery, i.e. Nettles
In a Stinging Poultice recipe they count as nettles and string
Would an alternative crafting window prevent the issue of it using all your nettles before any other string? Since you have to use a cauldron already maybe using the cauldron as the nettle input, or have a step to cook the nettles and make them actually not a string anymore?