Any joy? The only temporay fix I found is to restart the tv, it then picks up the arc, but hard to tell if this is a tv problem or soundbar.
Here's the story:
I called LG 2 days ago and gave them all the details, they tried to just say it's a TV issue and I need to contact my manufacturer (in my case, Kogan who I had already been going back and forth with for about a week at this point trying to get a fix assuming it was an issue with my TV which after seeing your post, now I'm think it isn't) but I mentioned how I read that you have a Sony TV and have the exact same issue as me with the same temporary fix of completely restarting the TV which is making me think it's a soundbar issue.
I eventually got passed to a "technical specialist" who went through some basic troubleshooting that I had tried plenty of times which included making sure the cables are hooked up correctly, making sure the TV sound setting is set to output through HDMI ARC, making sure it's actually plugged into the HDMI ARC compatible port etc. etc. The rep tried to tell me to just "manually select the ARC setting on the soundbar" which is something that I had to explain to him didn't actually exist since Optical and ARC share the same input setting and it only switches to ARC when it detects it. At one point they even tried to claim that the soundbar is only compatible with LG SIMPLINK, which is wrong for so many reasons... First, it's not even mentioned in the marketing or the instruction manual for the soundbar. Second, SIMPLINK is just LGs fancy name for HDMI CEC (same with BraviaSync on Sony, AnyNet on Samsung etc.) and third there's a special LG TV source input on the soundbar which makes no sense if the soundbar is supposedly only compatible with LG TVs.
Anyway eventually he took down my details and told me someone would get back to me within 48 hours. But yesterday at midday I got a text from LG telling me that they recieved my case and would get back to me in 48 hours, so I guess it's now 48 hours from yesterday?
I'll let you know when I hear back. I'm definitely thinking it's a soundbar issue since we both have completely seperate TVs from seperate companies and are having the exact same issue. You are literally the only person I've been able to find that also has this issue. So either very few people have actually bought this soundbar, or it's a very isolated problem.
Hopefully I hear back soon, and I'll let you know when I do. Maybe if you find some time to call as well and get it escalated enough to where they launch a case and "get back to you within 48 hours" they might take it a little more seriously? But I'll let you know what they say when I hear back.