Line created with draw3 overlaps fill3-object which should be in foreground. Using camlight solves the problem

I'm a little bit confused why this upper left line is in the foreground:

When I add

camlight;

after drawing, the picture is displayed how it should be:

It's to much code to provide here, but these are the last lines of my file:

for kn = 1:6
    plot3([L_P_Points(1,kn), U_P_Points(1,kn)], [L_P_Points(2,kn), U_P_Points(2,kn)], [L_P_Points(3,kn), U_P_Points(3,kn)], 'k', 'LineWidth', 3)
end
xlabel('x'); ylabel('y'); zlabel('z');
axis([-1.2 1.2 -1.2 1.2 -0.5 1.2]); axis equal
grid
view(20, 20)
%camlight

hold off

In the for-loop I draw these 6 lines. Maybe somebody knows this problem and has a solution?