Hi Mikl!
When it’s new and showing these kind of issues. Always use your warranty and get a new one.
Anyway, to answer this. It’s the battery which is having a defect or the battery it’s cable.
As for the Steam Deck OLED, the cable is fused with the battery, so you need to replace the battery to get it fixed.
Guide: [guide|168676]
But again,, since you still have warranty on it, return it and get yourself a new one.
Have a nice day!