It would be interesting to compare mfgr date/batch. And system details like Mobos, psu and gpus. Though it seems to be judging be @Timo.H1 's testing using an external usb soundcard that it is indeed headset specific. So either an assembly issue like maybe the sound wire routed incorrectly. Defective component or change in BOM in later built 8kX hmds.
How to read serial# by Dallas below.