Not sure, but I think effects based on position in Painter expect monochrome positions and probably only read the first channel (R) when you give them a color (all axis) position map. Until we provide a setting in Painter to be able to specify whether the input is color or grayscale, you'll probably have to rebake your…
I'm finding that if I bake a position map (all axis) in Designer and pop that into painter, an effect like ground dirt reads the "bottom" as -X to X instead of -Y to Y. I've worked around it by pre-emptively rotating my mesh in Maya before baking in SD, or just setting the axis to Y in the SD mesh baker, but I am curious…