Diagnosis
An external device connected via HDMI-CEC is sending a wake-up signal, or a scheduled function is enabled.
How to Fix It
First, disable HDMI-CEC. Go to All Settings > Connection > Device Connection Settings > HDMI Settings and turn 'SIMPLINK (HDMI-CEC)' OFF. This stops other devices (like a game console or soundbar) from turning the TV on.
If That Didn't Work
Check for automatic updates or timers. Go to All Settings > General > System > Time & Timer. Turn OFF 'Auto Power On' and 'Quick Start+'. Also, in All Settings > General > System > Additional Settings > Power & Energy, turn OFF 'Auto Power Off'.
Heads Up
If you use the LG ThinQ app or have the TV on a smart home schedule (like Google Home routines), check those external apps for any 'turn on' commands you may have set accidentally.