Point defects can seriously affect the performance of scintillator materials. Therefore, the fabrication of defect-free scintillators is an exciting goal for materials scientists. By applying an advanced optical ceramic preparation process, $({\mathrm{Lu}}_{1-x}{\mathrm{Y}}_x{)}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}:\mathrm{Pr}$ ($\mathrm{LuYAG}:\mathrm{Pr}$) ceramics free of defect-induced host luminescence are produced. Moreover, a detailed low-temperature thermoluminescence (TSL) study reveals that all TSL peak intensities and trap depths decrease in the Y admixture. The whole phenomenology indicates that a joint effect of “band-gap engineering” and “defect engineering” works in $\mathrm{LuYAG}:\mathrm{Pr}$ ceramics, as confirmed by the comparison between TSL data, radioluminescence temperature-dependence measurements, and first-principles calculations. As a consequence of material optimization, the light yield of $\mathrm{LuYAG}:\mathrm{Pr}$ ceramics with 25% Y is found to be $24400\mathrm{ph}/\mathrm{MeV}$ with a short $1.0\text{−}\mu \mathrm{s}$ shaping time, which is 20% higher than the value obtained for Y-free $\mathrm{LuAG}:\mathrm{Pr}$ ceramics. We close with an explanation of the role of Y that also suggests future research directions concerning other oxide scintillators.