The moveInXXXX commands are instant, which means
that it's your trigger that fires early, start debugging there.
Could it just be that you killed off the convoy quicker with two players
than just one, thus satisfying the East not present trigger earlier than you use to.
On another note;
if all units are in the same group as the player, use Foreach instead:
"_x moveInCargo truck1" foreach units group player
Makes changes easier.