Hi Jonah,
I'm Rodrigo and I'll help you.
When the Bluetooth headset is connected and the microphone is in use, it enters hands-free mode. The hands-free mode has this lower audio quality.
To avoid this, you can disable the hands-free mode or disable the microphone to avoid entering the hands-free mode.
To disable the microphone, with the Bluetooth device connected, go to Settings > System > Sounds > under Advanced "More sound settings" Recording tab, right-click the headset device > Disable.
To disable the hands-free mode, go to Settings >Bluetooth & Devices > Devices > More devices and printer settings > double-click the Bluetooth device > Services tab > uncheck "Handsfree telephony"