I have a similar issue. The left earcup does not work unless I angle the hdmi cable at a certain angle. It appears to be a loose cable. Unfortunately the small store I purchased from has gone bust during covid and I now can't take it back. Sadly ASUS customer support has said that his is my only option and I'm now stuck - fool me for not registering :S
I've taken the cup apart so that I can look inside and it's caused by movement when on the inside of the rubber joint, just as the cables are soldered to the board.
I believe that it's a design fault and it's really disappointing, especially from a reputable seller like ASUS. I've been looking on Ebay for a replacement headset and I must say that I've seen two in the last week that have exactly the same fault!
I'm going to try to get this one fixed as I can't deny that the sound quality is, in my opinion the best on the market but I'll never trust another ASUS product after this. Unfortunately, I'd not recommend the headset to friends either. It's too much to lose after a year or two of use.
I'm going to continue this thread and recommend anyone I find to contribute. I'm in UK and I'm going to look at consumer rights, particularly as to whether or not this can be raised as a potential design fault. If you know anyone else with this issue then please get them to comment here:
https://rog.asus.com/forum/showthread.php?108904-HDMI-cable-after-4-monts-already-broken