Like @DMASSO mentioned, is it possible that one of the leads coming from the forward direction output on the board is pinched and grounds out somewhere, while the other lead is fine? Then when the engine tries to run in one direction, only one motor is working (which, due to the motor worms, might keep the other motor "locked") and giving the illusion that it's fighting the working motor and running backwards, but not actually running at all?
But in the other direction, either the pinched wire gets pulled or tugged slightly in a way that it loses its short and allows "normal" running, or the opposing worm is allowed to turn in that direction even though it's not being powered, giving the illusion that both motors are now turning?
@RLaHaie, have you actually tried to apply power to the rollers with the engine off the track to see if both motors are indeed running, but one in reverse? I agree with @GGG , that would be a very unusual scenario and hard to imagine how that would happen (even with the farfetched pinched wire scenario above). Maybe you need to confirm the engine is actually doing what you think it's doing but off of the track.
Do you have gator clips-jumper wires? You can put the loco on a bench (protect it in a cradle made from a towel, good tip from elsewhere ion this board). Clip one gator clip to the center rail and to one of the pickup rollers. Clip the other clip to an outside rail or the ground post of your transformer and then to somewhere on the metal trucks of the loco. Start up the loco as you normally would, and actually watch both sets of wheels to see if both motors are behaving the way you think they are. Video would be good too!
