Log([O III]/Hβ) in the DIG as a function of log(Σ_Hα/erg s⁻¹ kpc⁻²) for different radial bins for individual galaxies in the PHANGS–MUSE sample. Galaxies are ordered by stellar mass from lowest (top left) to highest (bottom right). Several galaxies (NGC 3351, NGC 1300, NGC 1512, NGC 3627, NGC 1433, and NGC 1365) show exceptionally high line ratio values in their central regions (in red). Galaxies present a systematic change in the slope of the log([O III]/Hβ) versus Σ_Hα relation, with low-mass galaxies showing comparable line ratios at low and high Σ_Hα and high-mass galaxies showing a clear decrease in log([O III]/Hβ) with Σ_Hα. Both of these trends are at odds with predictions from photoionisation models.