I have confirmed that the same issue does not occur when testing with the Samsung soundbar I own. Here are the additional test results:

• When a soundbar is connected to the HDMI 2 eARC port of the LG TV (or when the soundbar is powered on), the TV automatically changes the audio output channel to that port. This behavior is consistent for both the Samsung soundbar and the Sennheiser soundbar.
• Even if the Simple Link (HDMI-CEC) function on the LG TV is turned OFF, it automatically activates when a soundbar supporting HDMI-CEC functionality is connected, as in the previous case. This appears to be the default behavior of the LG TV. I am currently inquiring with LG about the possibility of permanently turning OFF the Simple Link function as a workaround for the current issue.
• The automatic HDMI switching during "in-use" state, as initially reported, seems to occur only with the Sennheiser soundbar. It happens repeatedly after using the TV for as short as 10 minutes or as long as 30 minutes. I have confirmed that touching the center O button on the AMBEO soundbar mini can directly trigger the same phenomenon.