Is this dip deterministic, i.e. if you run the simulation again, it has the exactly same magnitude?
You haven’t posted your files (see Please Read This First: Guidelines and Suggestions for posting LAMMPS questions to enhance your chances to get answers), so I cannot reproduce your simulation. Therefore, the only other piece of advice I have is to output value of v_pull_force through thermo_style custom command and see if the value abruptly changes between runs.