All three of your digitizers seem to have the polarizing filter. Do you happen to have bought a spare one, or do you remember if it was completely transparent or a bit dark?

A definitive test will be to use two digitizers or one digitizer and one polarizing filter/polarized glasses and look through both of them while turning one of them around the axis perpendicular to it's surface. You should notice that in a certain relative angle (x) no light passes through (completely black) and at an exactly perpendicular angle (90° + x) the darkness of both glasses is about the same as one of them.