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

Try to recalibrate the battery. Enter battery storage mode https://help.steampowered.com/en/faqs/view/69E3-14AF-9764-4C28 and then do a full charge/discharge cycle.

If that doesn’t help, go to UEFI setup and keep this screen open until the battery runs out.

If that also doesn’t help, then contact Valve support to arrange RMA.

I’m clearly not the manufacturer. Contact them.