That's the problem with hunting down erratic bugs. There are a large number of parameters to vary in order to find the combination that makes the problem appear with some frequency. In Sunday's game, the French had 7 players, 1 corps, 4 division and 2 brigade commanders. All of my messages that bounced, except one, were to division commanders. Was the test game you ran at brigade level couriers? I have certainly noticed that with more humans in the game, the frequency of the bug increases. With just 2 player, I only see it occur once per game. This happens with both a stock and Nappy OOB.
This bug is analogous to the time bug. Some variable is accidentally reset and the order is not executed properly. Maybe fixing the time bug, moved the problem to the next variable in the list. I'm grasping for straws here.

I can make this march and I will make Georgia howl.