Texture Painting - Textures Switch Constantly

While painting an object with multiple textures as layers, if I change the brush size or strength the texture selected in the texture slot always changes to a different texture.

I must go back to the texture slot and pick the texture I want to paint on over and over again each time I adjust settings. Very frustrating and I haven’t been able to find anything online about this specific problem.

Is anybody else experiencing this? It has been happening for months across multiples 2.9x versions.