The aTV sends audio over the HDMI cable to the TV.
The TV then routes the audio to the speakers you want.
The aTV doesn't really care what speakers the TV is sending audio to. The TV will tell the aTV what the speakers are capable of (stereo, surround, atmos, etc...) but that's about it.
it probably resets on when you turn the TV back on because it does hardware handshakes when it turns, everyone says hello to each other, and the TV sees the soundbar and assumes you want to use the extra gear you have.
I have an LG CX, not sure what the current model's menus look like, but I press the menu button on the LG remote, I get a quick menu down the left side with a few items, you can customize the items in the quick menu, one is audio output, you can select and it will let you scroll through the options.
If you're decently technical, you can use homebridge, and the webos plugin to change which speakers you're outputting to. (If you need more info than that, then this probably isn't a good solution for you)