MacBook Air Shut Down During New Battery Calibration

I've just replaced the battery in my wife's MacBook Air 2017 with a new one from iFixit.

Installation was easy. I followed the instructions to calibrate the new battery - charged it to 100%, kept charging for a few hours, removed the power and started using it in order to fully discharge.

After a couple of hours of use I put the laptop to sleep by shutting the lid (I had stuff to do). When I went back to it a few hours later I found that it had shut down while asleep.

Battery was still above 80%. When I looked at the logs (powerd log I think) it said that the shutdown reason was code 0 Battery Disconnected. Which it obviously wasn't. Since logging in again (still running on battery) I've used it flawlessly but only allowed it to sleep with the lid open.

I'm still in the full discharge process - so the battery is still working with no problems. Any idea why I had that strange shutdown?