Seeing as this happens after unlocking (so after pressing the power button,) it sounds like a hardware issue, specifically with the power button. A quick test is to feel how ‘clicky’ your power button feels - if it feels spongy, it may be broken. If you’re not sure, you can open the device up and see if everything’s in place.
There’s a thread here talking about the same issue you’re having, and the possible fixes and links to some other threads. There’s also some pictures of how the internal part of the power button should look.
If it does turn out to be a power button issue, it’s either a matter of finding a replacement switch and a repair shop that will do the microsoldering for you, or replacing the board.
Edit: Urs beat me to the punch.