You lock the missile on a target while aiming it. The seeker will remain locked to exactly that target until it hits. Only way it's hitting a neighbouring truck instead is if that's what you aimed at when you pressed fire.

Ok. I suppose this works also if the tank moves -- but if it gets out of sight, let's say a truck drives in front of the tank, then might the javelin miss?

Or is the javelin so high up in the air, so it could typically see the tank anyway?

Yes, in principle, obstructions could make it miss. But also yes, Javelins fly lofted arc trajectories, unless you take the shot a half a second before the target is obstructed, the missile will almost certainly be able to maintain lock even if something obstructs your view of the target.