Interesting update:

I made separate files of the RT and midi 12-bar chart, and then listened to them in RealBand. First, both played flawlessly. Second, in each case the melody track was approximately 18 bars long and contained notes stretching beyond the 12 bar ending. I have no idea where these came from, but they are there in RB. Deleting them in RB made no difference: in each case, the triplets were replicated correctly.

Someone, please explain what the heck is going on here? I will copy and paste the 12 bars to a new file and see if that cures the problem, whatever it is.

Bob