Countless stories exist of non-functioning ARC, even w/ gear supposedly 'compatible' and made by the same manufacturer. It is (currently) a half-baked functionality at best.
Stick with the digital optical cable. It's a more reliable way to go, as you have discovered.
I have an LG 65UH770 TV and an LG SH4 soundbar (both ARC compatible). I bought a new high speed HDMI cord with Ethernet to utilize the HDMI ARC function. I plugged the cord into the HDMI ARC out port on the sound bar, and into the HDMI 2 ARC port on the TV (I checked all cords to ensure they were connected). Then, I went into the sound settings on the TV and changed the sound out setting from internal TV speakers to Audio Out (Optical/HDMI ARC). No sound. I tried the same method but I removed the HDMI cord and used an optical digital cord instead and sound played right away. Any idea why the HDMI ARC function is not working?