Diagnosis
An external device connected via HDMI-CEC is sending a 'power on' signal, or the TV's built-in Fast Start feature is malfunctioning.
How to Fix It
Disable CEC and Fast Start.
1. Go to Menu > System > CEC. Turn OFF 'System Audio Control' and 'One-Touch Play'.
2. Go to Menu > Admin & Privacy > Power Mode. Change it from 'Quick Start' to 'Eco Mode'.
3. Unplug any devices from the HDMI ports, then plug the TV itself out of the wall for 2 minutes to clear any residual charge.
If That Didn't Work
Check for scheduled events. Go to Menu > Timers & Clocks. Ensure 'Auto Power On', 'Sleep Timer', and any other schedules are set to OFF.
Heads Up
Eco Mode will make the TV take 15-20 seconds longer to turn on, but it completely cuts power to the mainboard when off, preventing phantom wakes.