i have such a problem - no sound from central channel from time to time.
i think there is some problem with lpcm output from ATV4K or HDMI handshaking with HD621.
after some manipulation all is ok, but after HDMI cables reconnection the problem comes back.
i have no algoritm to fix this bug. sometimes changing MD1 to MD2 helps, sometimes HDMI extractor on/off helps.

HD621 , 800v4, 861v4, DSP7000, DSP5200HC