Keep iPad always on and locked to one app. In >iOS 12
I have seen several people with the same issue. Assuming you upgraded the iPad with guided access enabled follow these steps:


  I have found that if Guided access was enabled prior to the upgrade it
  might reset in this process.
  
  
  Turn off Guided Access in Accessibility / Settings.
  Hard reboot.  (Hold Home and Lock buttons for 7 seconds)
  Turn on Guided Access in Accessibility / Settings.  Reset Passcode.
  




From Kiosk:


  iOS 12.1.1 has been released and includes a fix to this issue, but requires a change to a new setting introduced in this release to prevent the screen from sleeping: General > Accessibility > Guided Access > Mirror Display Auto-Lock > ON.  


to update your device, go to Settings -> General -> Software Update