Diagnosis
This is almost always caused by the TV's "Auto Device Detection" feature misbehaving. It mistakenly thinks a new device (like an antenna) has been connected and switches to it.
How to Fix It
You need to turn off the auto-switching feature. Press the Settings button, then go to All Settings > Connection > Device Connection Settings. Turn OFF "Auto Device Detection." Next, go to General > System > Additional Settings > Live TV. If you don't use an antenna at all, you can select "Live TV" here and turn it OFF entirely, which will remove it from your input list. If you do use it occasionally, just leave it off in the first step.
If That Didn't Work
There might be a signal issue with the antenna coax cable itself, even if it's not connected to an antenna. If you have a loose or unconnected coax cable plugged into the TV's ANT/CABLE port, unplug it completely. A floating cable can pick up electrical noise that tricks the TV into thinking a signal is present.
Heads Up
If you have a soundbar or receiver connected via HDMI ARC (the HDMI 2 port on this model), its CEC control might also be sending switch commands. Try turning off Simplink (HDMI-CEC) in the Connection settings as a test to see if the unwanted switching stops.