Iam having the same issues with the LG C9 when using Dolby Vision on HBO Max through a Chromecast. I also have a Bose soundbar. I decided to try and watch Dolby Vision without the soundbar as the sound option and it worked flawlessly.
As a workaround, I then changed the audio back to the soundbar and I'm still having no issues. Annoying but at least a temporary workaround