I have learned that stopping potential is the voltage at which the current becomes zero. I also learned that the stopping potential is independent of intensity. Knowing both the stopping voltage and the corresponding wavelength you can determine a materials work function ($\phi$) using the relation $V_se=(hc/\lambda)-\phi$.
Using the photoelectric effect phet sim I am trying to solve for a certain materials work function. I found that the stopping voltage for sodium with a wavelength of 367nm is about -0.95V as shown below. However, after increasing the intensity (without changing wavelength or battery voltage) the current increases. I then need to lower the voltage to -1.01V in order to bring the current back down to zero. This seems to contradict what I have learned about stopping potential being independent of intensity.
I understand that increasing intensity will increase the amount of electrons ejected. But below the stopping voltage no amount of intensity should induce a current.
Am I measuring stopping potential in the wrong way? Solving for the work function using -0.95V and -1.01V yields two different answers.