First suggestion - RMA it

Second suggestion, if you wanna try to fix it yourself either in the meantime or before RMA:

• Play games on it until it completely runs out of charge
• Launch into the bios until it completely runs out of charge
• Repeat the above until it no longer turns on
• Plug in your Steam Deck. If it turns on automatically, repeat the above two steps until it doesn’t when you plug it in
• Leave it charging and turned off for 6-8 hours, preferably overnight, then turn it on and check again

If you’re back up to 95%+ then it’s a battery calibration problem which most are