I would agree that it's not a good idea to handpaint normals, but sometimes fixing a minor normal map error by blending between 2 maps is way easier than figuring out a different way to model or bake it :(
you have your answer right at the end of your post - fix your bake. I forget exactly what painter is doing here - I've been through this with a number of artists over the last few years but it's been a while. IIRC its partly down to what gets picked, partly the color space transforms applied to picked/painted colors and…