Baking soda + oil solubility
The baking soda grain size might be bigger in this new brand. When you sift, you break up individual grains that have clumped together due to moisture or being pressed together tightly but there is a fundamental particle size from how the baking soda was manufactured that would require further processing to reduce (e.g. milling). The larger grain size would adversely affect the solubility. It could also be that the baking soda you buy now is either missing some additives or has some additives that are detrimental to your final product.