If you take too long, the map exapands. There's timer to prevent just pilling up units. Well the trucks will run away and once they're offmap, the map should expands. iirc its around 8min to do the first part.
I wanted the mission to be short, that's why I picked a small map, and the other half can't be used, cause it's symetric and I don't like it for missions.
Some additional reinforcements for finishing the bonus objective is a good idea, I might add it.
And no arty is exploding at drop, everything works fine there.