Diagnosis
The CEC (HDMI Consumer Electronics Control) function is active and interfering. Your TV is sending a wake-up signal via the HDMI ARC cable.
How to Fix It
Go into the soundbar's settings menu. Disable the HDMI CEC function. On the remote, press the Settings button. Navigate to 'HDMI CEC' and set it to OFF. This stops the TV from taking control of the input selection.
If That Didn't Work
If the setting is already off, the issue may be a faulty HDMI cable. Replace the HDMI cable connecting the soundbar to the TV's ARC port. A damaged cable can send phantom signals.
Heads Up
Disabling CEC means you lose the ability to control the soundbar volume with your TV remote via HDMI. You'll need to use the soundbar remote separately.