Just picked up mine (12 Pro, sorry to the above) and the guy in the store wanted me to open and test it there and then. 0% battery (as heard about before here) - confirmed on his phone as well as mine.
Weirdly, we plugged it in to one of the display iPhone lightning cables and he said wait 5 minutes. I was impatient and waited only 3. I checked it and it showed over 20%. I don’t believe it really charged 20% in less than 5 minutes.
I can only think there’s some calibration that goes on after it is connected to power for the first time, and it might be best to wait a week or two before really measuring its performance?
Mine said 39 percent out of he box. Could just be luck of the draw, as you said, software taking a minute or two to "recalibrate" for the new battery to get an accurate reading.