I stumbled across this thread when I recently hit a similar problem with my new Denon AVR-S660H receiver + LG 55C3 TV and thought I would register to post just so that this model number now also comes up in search results in case it helps someone else.
I have the current stable version of libreelec installed on a brand new Pi 4B and I tested it worked ok with my TV by connecting directly to one of the TV's HDMI ports but when I tried it with my receiver I got no output.
In the end I resolved the problem by changing from the 'Media Player' input on the receiver to 'Aux 2' (as some, but not all, the posters in this thread have found). On my particular TV I note that the Aux 2 port is marked as being '8k' capable whereas the Media Player port is '4k' (although I've no idea whether or not that is relevant to this problem).