It might be a good thing you updated the firmware. What if you actually have a faulty battery pack?
You've already proven it's not the charger.
It could be a bug in the firmware but why not have the battery replaced under warranty regardless.
Interesting. The old firmware "seems" to be calculating Wh correctly. 94% of 700Wh is 658Wh.
The updated firmware is doubling the loss in health. In your case a 6% loss doubled to 12%. 700Wh x 88% is 618Wh
What does @Specialized RnD says?
Mine just displays battery heath as a percentage. If the battery health percentage dropped after the update then that's actually a good thing. That would prevent overcharging and other oddities.
The fact that the battery may actually be prematurely unhealthy is another issue.