Does your iPhone register a battery level for the right AirPod? Here are two ways to check:
1. Put both AirPods in the case and close it. Unlock your iPhone and leave it on the home screen. Bring your AirPods case near to the iPhone and open it. It should display the battery percentage for both the AirPods and the case:
View attachment 1935845
Now remove one of the AirPods from the case and leave the other one in the case. It should then show the individual battery levels*:
View attachment 1935846
*If one is lower than the other, I think it will show both percentages even if they're both in the case still.
2. Add either one of the battery widgets that show the percentages and leave one AirPod in the case:
View attachment 1935850