Hi there. You should have a proximity sensor that should turn the screen off when you raise it to your face. I found this post in the Lenovo forums and the moderator suggests testing it in safe mode.
The process for turning on Safe mode is as follows:
Press and hold the Power button.
Tap and hold Power off.
When the Reboot to safe modeprompt appears, tap again or tap OK.
Your device will then restart and it will say “Safe mode” in the bottom-left corner.
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https://forums.androidcentral.com/showthread.php?t=409154