Our 2010 Hyundai Santa Fe had pearl three step paint, the summer when washing it I found this exact same thing you did with your Mazda. Subtle difference and not something you notice from a short distance away, but when you are a car guy who washes your car by hand you notice these things.
Turns out the rear bumper is actually painted a different colour altogether, it actually didn’t have any pearl in it. This was done on purpose from factory, likely cost cutting but also if you have any little fender benders it’s much cheaper to paint.
In the end, I accepted it as it wasn’t something others noticed and it is a CUV and not some high end sports car where my level of care would be much higher for such things.