Seems like there's a big problem with the way this TV handles HDMI-CEC with receivers. If you only have a single device connected to the receiver, it will work fine, but whenever you have multiple devices connected, then no matter what you do, the TV will force the first device listed under the receiver input to turn on when switching to that HDMI input on the TV. The only way to bypass it seems to be to manually select the device in the list under the HDMI port.
Otherwise, if you simply select that HDMI port, it will automatically turn on the first device. If you turn on one of the other devices, that will trigger HDMI-CEC, which will cause the TV to switch to that HDMI port and then, again, turn on the first device.
This is incorrect behavior. When none of the receiver's devices are manually selected by the user on the TV's input menu, the TV should only switch to the HDMI port and display whatever is already there.
For example, on my receiver (connected to HDMI2), I have my PS4 and PS3 connected. The first device listed is the PS4, so even if I turn on my PS3, the PS4 turns on and then the receiver switches back to the PS4 input. If I turn off CEC on the PS4, it just automatically turns on the PS3 if I switch to HDMI2. I'm planning to buy a new 4K Blu-ray player soon, and so this will cause even more problems. I really don't want to turn off CEC, because it can be quite useful, but LG needs to fix this behavior.
EDIT: Just sent this message to LG along with this image
. Hope they can fix this.