There is something strange going on there. I have 2 Rail DCC and have installed decoders in more than 50 locos. I have never had one do that. I have never used a Digitrax 126 decoder, but I have used a few of the DH123 decoders. They are work fine.
Now your description may have confused us. When you put the loco on the track the first way, does it go forward? Then when you turn it 180 degrees, does it still go forward which is now the opposite direction from the first setting? That is the way it is supposed to behave.
The programming defines the forward direction to the decoder. The decoder than puts out DC to the motor to go in that direction, whenever it gets a signal to go forward.
ON straight DC, changing the loco 180 degrees on the track, changes the polarity to the loco and it goes in the same direction as went before.
