This is a Bluetooth issue, not Linux or Deck specific. When microphone is enabled on a headset, Bluetooth will switch to a different audio codec. That codec supports one output channel (microphone) and one input channel, so it will be mono audio. The sound quality is good enough for audio calls but not for anything else. The only thing you can do is to disable the microphone. Or, you can do what people have done for couple decades now: complain at Bluetooth Special Interest Group until they improve the standard.