The small rounded leaves are known as "cotyledons" and their only purpose is to protect the growing tip as it pushes through the soil. After the seedling sprouts, their job is done, so they generally turn yellow, die and fall off or shrivel up anyway, which is all perfectly normal behaviour for cannabis plants. The cotyledons doing so will not affect anything and it is not a sign of disease or deficiency or anything you have/have not done, they just die as part of the normal growing process.