It is a bug that was corrected in the latest version of TS12 (I do not know if MAC versions were corrected)
Trainz Mac 1 = not fixed (TS10 port)
Trainz Mac 2 = fixed (TS12SP1 port)
Can confirm this for myself, having gotten the SP GP9 units back when I was transferring from Mac 1 to 2. Worked without issue in the latter, got the same bug as OP in the former.
Cheers,
SM