Turn off CEC (Simplink) before you switch to the internal speakers.

Thats disappointing. Turning CEC (Simplink) off means I lose that functionality between my other devices hooked up to the TV

And yet turns on the internal speakers, per the CEC specifications. Feel free to blame the consortium of companies that came up with the ‘rules’ for CEC, which only allow one audio sink at a time. 

Is BEAM’s Night Sound useful? 

Another approach is inserting a CEC blocker in the HDMI cable to BEAM. Note that this will also block Volume commands on CEC. You’ll need to enable IR Volume control of BEAM. And, there may be some LG quirks since it cannot use CEC.

It must be Sonos implementation, my old Bose soundbar didn't force back ARC when changing to Internal Speaker.

• Sonos has a player led HDMI Arc handshake. (The sound bar initiates a CEC connection with the TV) 
• Bose has a TV led HDMI Arc handshake. (The TV initiates a CEC connection with the Soundbar) 

I was just asking if there was a way to switch implementations which I see there is not.

Were you using this same LG TV with the BOSE system?

Yes it is the same TV.

A sometimes helpful procedure is to remove all HDMI connections from the TV, then reboot the TV and BEAM. Now, connect BEAM to the TV and let them renegotiate. After the TV and BEAM are finished negotiating (give then a couple minutes), then connect any other devices to the TV.

If this clears the issue a future update might result in this or another issue, and it will always be the “we’ve never seen this issue before” (it must be the other guy’s fault) response.