I have a Bravia XR65X95K and a Denon receiver. I’ve got it set so that I control everything with the Sony remote. There is a little lag when changing the volume but it’s always in sync if that makes sense. I’m not at home but I can show my settings later if that helps. Maybe post yours and I can tell you what’s different.
Could this be because you’ve set the forwarded address to be insecure in the reverse proxy essentially doing SSL termination at the reverse proxy? To test this theory, set the destination local address to also use https not just http