You can also verify LAMMPS gives the correct behavior for slab systems yourself by comparing to an analytic solution. Just place two oppositely charged particles in a slab system. They form two infinite sheets of charge due to periodic boundary conditions in the x and y directions. If the charges are far apart enough in the z-direction, the force in the z-direction converges to 2*pi*q^2/(Lx*Ly), (see Understanding Molecular Simulation, Frenkel and Smit, p. 318), and LAMMPS reproduces this result * only if* you include the correction factor given in Eq. 12 of the paper.

Stan