I had the same problem,
Primary problem is that these batteries are so tiny that they will charge faster and as we all know whenever these Pods are in case are always on charging mode and due to this batteries deteriorates faster. Apple don’t seem to have any solution yet. (I recommend keep out your Pods from the case whenever it’s not in use.)
For me I cleared all the dust from the case and then parity was restored. Dust causes charging malfunctioning and you’ll always see charging differences from one Pod to another.
I hope this is helpful. :) Good Day!!