Nice soundbar! My idea is probably too simple. When setting up my LG TV & LG soundbar I couldn't get any sound via ARC either.
My soundbar has 2 HDMI connections one marked sound in and naturally, I connected the TV ARC port to the soundbar port port marked Sound In. I couldn't get any sound unless I used the optical connection. To my surprise, I found that if I used the other HDMI port on the soundbar it worked perfectly! This still seems wrong to me but everything works and sounds good so have not made any inquiries.