[S III]/[O III] vs. [S III]/[S II] diagnostic diagrams introduced by Stasińska (2006) showing the density contours for all the star-forming spaxels of our sample (in black), with superimposed Dopita et al. (2013), Levesque et al. (2010), Byler et al. (2017) pdva+geneva, Byler et al. (2017) MIST, and Pérez-Montero (2014) models, respectively. Metallicity increases horizontally from yellow to green, while ionisation parameter increases vertically [redder means higher log(q) and bluer lower log(q)]. The red contours correspond to the brighest MaNGA star-forming regions [EW(Hα) > 100], while the grey dots are the H II regions analysed by Kreckel et al. (2019) with the PHANGS survey. Current photoionisation models fail to reproduce the observed [S III] lines.