Student evaluation of teaching (SET) is important for assessing university instructors' performance. However, this system seems biased as students' grade expectations result in rewards or penalties in SET. As a fair evaluation of grades became difficult during the COVID-19 pandemic, universities implemented a relaxed grade policy that expanded the distribution of high grades. This grade inflation altered students' expected grade. Through empirical analysis, this study examined the change in the relationship between bias and SET due to grade inflation. A top-ranking South Korean university provided 125,003 cases of SET data in 2019 and 2020 for the analysis. Grade inflation diminished the biasing effect on SET, mainly in terms of reward. Furthermore, the group with the lowest grade point average (GPA) showed the highest decrease in rewards, and the group with the highest GPA showed maximum decrease in punishment. This finding implies that a change in expected grades due to factors other than lectures may alter students' attitudes toward SET, and grade expectations may play a key role in reducing bias in SET.