This does seem to be normal HDMI-CEC behaviour, though I can see it is irritating. Most people get by by temporarily turning off CEC, though LG seems to turn even that back on automatically.

There has been talk that this behaviour is more serious with Sonos devices than with other manufacturers’ devices, but this has not been proven.

This is a “feature” of the LG.

As a work around you can try disabling SIMPLINK in the LG or adding a CEC blocker to the HDMI cable between the TV and BEAM. There will be some operational trade-offs, such as less integrated Volume control. You’ll need to switch to IR control of BEAM’s Volume.

In fairness to all of the designers, it is reasonable to assume that if the user connects a soundbar, the user would prefer to play TV audio through the soundbar, rather than the TV speakers.