Sounds like a stuck button. Personally I would disassembly the device and have a look at the button and surrounding parts for any damage or liquid or debris at all. If there is any physical damage then an RMA or maybe replacement parts can be ordered (like you can buy replacement rubber if that feels worn at all). Otherwise I would ensure everything is clean and free of and liquids, stickiness or debris then reassemble the device. Even if nothing looked wrong I would test it again and see if the act of disassembly did something to solve the issue which it sometimes does for things.

IFixit has guides for the [LCD (www.ifixit.com/Guide/…/148933) and OLED versions and overall the steam deck is not very hard to disassemble compared to other small electronics. Though if you are unsure about this you may just want to talk to valve support first. If you accidentally damage something that could affect your ability to get an RMA.