The AirPods charging case is prone to battery drainage issues
Apple is already handing out replacements to affected customers. One described getting a new set of AirPods which didn’t lose charge over night. Chances are that the battery issue won't turn out to be a widespread problem, perhaps contained to an early batch of units. However, at this point it's not exactly clear how many users are affected, or if the issue could resolve itself over time as charge cycles add up.
If you find yourself experiencing battery drain issues with the AirPods charging case, your best bet for now will be going to the Apple store for a replacement.