Liu et al. (Dalton Trans., 2020, 49, 3615–3621) reported that emission of the Na₃Sc₂(PO₄)₃:Eu²⁺ phosphor showed tunable thermal quenching properties which were ascribed to the thermal event associated with polymorphic phase transformations. In this comment, I argue that the abnormal negative thermal quenching of the Na₃Sc₂(PO₄)₃:Eu²⁺ phosphor could be a pitfall caused by a diminishment in the optical path lengths of the spectrofluorometer originating from a continuous increase in the volume of the phosphor due to lattice thermal expansion and phase transformation at elevated temperatures.