I stumbled recently upon the same issue, that makes the game crash when approaching these "round" convoys. I figured out, that when I deactivate the Waterstream+Exhaust Combi v.2.3 for GWX3 mod, the game crash no more in vicinity of these "round" convoys.
So, every time I get a radio message of "the round convoy" and I want to attack it, I make game save and exit the game to go deactiviting that mod. After the convoy is again far away, I can reactivate that mod, cause it causes no other problems(I hope).