It's a very weird issue, after i hold my phone with the screen down it suddenly locks it self and the screen goes off without any reason. And no i did not pressed the power button while im holding it.
The only documented example of this I could find was in Sony phones. Turning off Stamina fixed it for this person. They also recommend clearing the cache for Stamina
- Turn off Stamina mode from Power management.
- Go to Settings -> Apps -> All -> Stamina mode -> Force stop, Clear data and restart your phone.