When someone in cosmere is compounding they are not producing Investiture but it's more like they pull investiture from spritual realm by using their metalmind as a key. Metals acts as a key for pulling investiture from spritual realm and i think when the key have some latent investiture of it's own it is able to pull more investiture from the spritual realm. So the first law of thermodynamics holds. In cosmere the conservation of mass and energy as well as first law of thermodynamics are relevant.