The phone locks it self while it lays with the screen down 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.