I have an old 4K LG from 2015 that works fine with my Yamaha, EXCEPT for no sound (video stream is fine) from the FETCH Mighty when turning on the TV. Luckily the Yamaha RX-V385 AVR has SCENE buttons on the remote, and I set up the TV button to all the correct inputs, outputs, and sound to recognise the FETCH box properly. I use a tweaked DPL-MOVIE decoder set up for the TV scene button. It is 1 extra step after turning on the TV, waiting for the AVR to initialise, then turn on the FETCH box, then the scene button press, but we are used to that now.
This could well be a timing issue with the firmware (TV or FETCH box), as NO software is 100%.