DIAGNOSIS This is almost always caused by a setting called 'Fast TV Start' (or a similar quick-resume feature) or by an HDMI-connected device sending a wake-up signal.
FIX Go to Settings > System > Power > Fast TV Start and turn it OFF. This feature keeps the TV in a low-power state instead of fully off, and it can get glitchy. While you're there, also turn off 'CEC' (often called 'System Standby' or 'HDMI Control' in TCL menus) for all your HDMI ports. This stops your game console, cable box, or soundbar from accidentally turning the TV on.
IF THAT DIDN'T WORK Check for a scheduled update or power-on timer. Go to Settings > System > Power > Power On Timer and make sure it's disabled. Also, in the 'Channel & Input' settings, ensure 'Auto Power On' isn't set to a specific input.
HEADS UP If you have an Alexa or Google Home routine that involves the TV, double-check it. A stray voice command you don't hear could also be the culprit.