I had the same issue, with the battery icon and single red bar, when I updated to iOS27 on my Gen 9. I was able to fix the issue, by using the Apple 20W charger in a wall socket. I plugged the Ipad in to the charger and left it alone. I did not try to reset it, start it, or anything else. I just left it alone. After about a day and a half on the charger, I pushed the power button to check it and I had 93 percent battery. It took that long for the battery and whatever else was going on to wake up. I kept it on the charger and got it to 100%. Ipad is working fine now, charging and staying charged. When I got it working, there was an update to iOS27 to fix bug issues. So Apple must be aware of it needing to be fixed.