COVID-19 introduced tremendous disruptions to the normal operation of transit agencies, led to a massive suspension of regular service, and disturbed the access to essential mobility for many transit-dependent populations. In this study, we reexamine the national-level service disruption of the public transportation system as a result of the COVID-19 pandemic and investigate the disparities in the reduction of transit supply, causing certain racial-ethnic groups to be disproportionately affected. To support our analysis, we collect GTFS data from 232 transit agencies covering over 89 million people across 45 states in the U.S. Our findings suggest more disadvantaged communities and certain racial-ethnic groups have experienced disproportional reductions in transit supply, regardless of their pre-pandemic level of service. Further, we employ causal mediation analyses to address the mediation effect of the pre-pandemic level of transit service on the relationship between race/ethnicity and transit supply reduction. This analysis validates the disproportionate reduction in service quality for specific racial/ethnic groups, a trend evident across the majority of transit agencies. In particular, Black Americans are found to be associated with the greatest absolute service loss and also the largest total effects on transit supply reductions, while the White and Asian groups are less impacted. We further report that the most impacted communities by transit loss mainly consist of minority populations, where the reductions in transit are correlated with pandemic-related health and economic burdens and result in greater changes in mobility intensity. Our study contributes to a comprehensive understanding of transit inequities due to the pandemic and provides valuable insights to better prepare public transportation systems against future nationwide disasters.