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Is this immediate? So you turn the screen and it just locks or you turn the screen and you know it will lock?
Does it show the lock screen or does it go dark, like you've pressed the power button, and when you wake the screen it is locked? I ask because it could be the way you're holding it or the way it is resting... Does it also happen if you rotate the other direction?
Is it plugged in when this happens or does it not matter it does it with or without being plugged in? If it happens when plugged in it could be that you have a bad cable and sensing an interruption in the charging cycle might be triggering this.
You could try the device in Safe Mode to see if it is being influenced by an app. These directions will help you get into and out of Safe Mode...
https://www.google.com/url?sa=t&rct=...sji0iS8Zcg02Jk If it does it in SM it could be a setting or a system bug. If it does not do this in SM it may be being tripped by an app. Let us know what your results are.
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