Is that the one where the convoy dips down into a river? Because I have one unit which always stops in the river with movement left . . . then starts back up again if I just hold position.
That's the only map where I've had this bug occur for me. However, it only occurred for me a long time ago when I was playing Roguetech, so I actually thought the bug was due to that mod (or one of the mods in it), and not in the base game. I've done that mission quite a few times since uninstalling Roguetech and never had it happen since (again, reinforcing my belief that it was a mod issue, not a HBS one).
Even without the convoy refusing to move, that particular map is super tedious. Its probably the most boring escort mission of them all (considering how long it takes to get the convoy to the drop off). And the Reinforcements on that map are so dumb. They either cannot figure out how to get over the big mountain to attack (either your mechs or the convoy) or if they do have jump jets, they typically jump in a way to expose their rear to some of your lance, making them easy to kill.
But yeah, I haven't had this bug occur in a really long time....definitely pre-Urban Warfare, maybe pre Flashpoint? Not sure. My current Escort Strategy is to completely ditch the convoy with all of my mechs. I get the annoying message from the convoy, bemoaning the loss of their escorts, followed by Darius telling me 'he's not wrong, commander' blah, blah. Yeah, but none of you dumb@sses are gonna get killed when I spawn the Reinforcements! Then, after the Reinforcements are killed, I move back and get the Convoy going again. If I keep one mech back to let the convoy keep moving, there's always one suicidal vehicle that zips far ahead and gets destroyed (although admittedly, if I kept a Mechwarrior with Multi-target, that might not happen to me).