I have Mi smart tv and I bought a soundbar some days back which is working fine with a Bluetooth connection but it's not connecting off HDMI ARC. When I turn on my tv it shows that the sound bar is connected but just in some seconds it disconnects. It shows that it is connected in the inputs list but it does not work.
Change the audio format on tv to PCM and try it , you can try to change the HDMI cable to another one, also turn HDMI CEC on your TV.