Some of these landscape photographers swear by this one, yet nobody
offers more than a couple sentences explaining how to use it.

I find that if used by itself part of the image gets a (usually
undesirable) magenta tint. Using a circular polarizer I can remove the
tint (while leaving the blue effect alone) in the viewfinder. Yet to my
surprise, when I look at the captured image on screen, the magenta is
still there! I'm really puzzled as how the optical viewfinder can be
inconsistent with the sensor.

Anyone can shed some light?