We present multiband photometric and spectroscopic observations of the type II supernova, (SN) 2019va, which shows an unusually flat plateau-phase evolution in its V-band light curve. Its pseudo-bolometric light curve even shows a weak brightening towards the end of the plateau phase. These uncommon features are related to the influence of Ni-56 decay on the light curve during the plateau phase, when the SN emission is usually dominated by cooling of the envelope. The inferred Ni-56 mass of SN 2019va is 0.088 +/- 0.018 M-circle dot, which is significantly larger than most SNe II. To estimate the influence of Ni-56 decay on the plateau-phase light curve, we calculate the ratio (dubbed as eta(Ni)) between the integrated time-weighted energy from Ni-56 decay and that from envelope cooling within the plateau phase, obtaining a value of 0.8 for SN 2019va, which is the second largest value among SNe II that has been measured. After removing the influence of Ni-56 decay on the plateau-phase light curve, we found that the progenitor/explosion parameters derived for SN 2019va are more reasonable. In addition, SN 2019va is found to have weaker metal lines in its spectra compared to other SNe IIP at similar epochs, implying a low-metallicity progenitor, which is consistent with the metal-poor environment inferred from the host-galaxy spectrum. We further discuss the possible reasons that might lead to SN 2019va-like events.