Ahila, thank you for you comment, that's an interesting metric to check! To do this I defined rare classes as the 20 COCO classes with the fewest labeled objects in the calibration split. Together, they had 773 eligible test detections. On this subset the ECE value for Temperature was about 9.89 percentage points, while Platt gave 3.74 (lower is better). So Platt still showed better calibration on the rare classes, although individual class results varied and many had small sample sizes.