The stability of the polarization state in Na_0.5Bi_0.5TiO₃ (NBT) ceramics has been a long-standing problem for its use in piezoelectric applications at elevated temperatures. It has been generally believed that the polarization state, depolarization temperature, and depolarization process are all linked to the grain size in these materials. In this work, we perform a thorough Piezoresponse Force Microscopy (PFM) study of the NBT ceramic samples with substantially different grain sizes sintered as a function of temperature. As-grown, macroscopically poled, and locally poled samples were investigated focusing on the polarization behavior at depolarization temperature. Switching Spectroscopy PFM (SS-PFM) measurements were conducted as a function of grain size and temperature. No direct correlation is observed between the grain size and the switching parameters in any sample. However, temperature-dependent measurements reveal significant differences that are explained by different concentrations of oxygen vacancies. We rationalized the observed behavior, e.g. apparent stabilization of the locally probed polarization above the depolarization temperature, by accumulation and depletion of oxygen vacancies in the vicinity of the internal boundary of the poled region. Significant asymmetry of the PFM hysteresis loops at elevated temperatures confirms this assumption.