This is your first post, so Please Read This First: Guidelines and Suggestions for posting LAMMPS questions. Also, please edit your post and enclose the input script in the preformatted text environment.
Said so, there is nothing wrong in your simulation. Ovito just wraps atoms at the periodic boundaries, which means that the grey atoms on top of the box have z < zlo.
If you want to visualise the slab without breaking, you need to either decrease the zlo value, or shift the whole system upwards, or use the unwrap modifier in Ovito.