I have a new tcl R635 tv that’s not hooked up to anything at all. I have my Xbox on the side of the room hooked up to a monitor. Whenever I turn on my Xbox my TV come on too. I also have the CEC unchecked on the TV.
Xbox has an IR blaster on it the shoots out commands to the tv in the room, instead of the much more common CEC protocol that would come through an HDMI cable. It's possible it just happens to match the code for the tv I guess? Go to your settings on the Xbox and disable it from turning on the tv when the system is turned on and see if that stops it. If not, no idea.