Similar has happened to me too in TR12 in the Mojave session, in DCC mode , but only after a had changed the locomotives from the original built ins. Possible then this does not occur in any standard trainz sessions . I noticed that it happens when locos are coupled facing different directions , ( strange - steam every time with me - incidental ? ), and I think they are dloads from other sources than the dls ) I can't recall whether it corrected after I changed them all facing the same direction, but I do remember that it corrected after I went out without saving and restarted.
The leading loco would be facing fwd but the pointer would work opposite when I start up ( portal problems perhaps ? ) , but then also whilst driving the train would suddenly slow down and when I check the DCC pointer is on the opposite direction to where It was.Then when I correct the pointer , it seems to correct itself a while later back to the normal direction and train handling is normal again.I might be barking up the wrong tree here guys, but that is what I found. Maybe it helps.
Jan