This is actually an artifact of the BT profile used. It is the same for most bluetooth headsets (all I know), and the same for all operating systems I know. Usually most people don't realize because when they enable the mic they're listening to... well, other people (definitely not music) and human voice sounds quite good at these bitrates. Good operating systems will just pause the music when you switch to the headset profile so that you don't hear it heavily compressed (until you finish the call and disable the mic).
It won't matter which "BT version" your adapter is using. 2.0, 4.0, 4.2 or 5.0. The headset profiles have remained unchanged for decades.
There is something new in 5.x -- the "LE Audio" profiles -- but I know no headset using it yet. And then there's the myriad of propietary extensions. Most of them are snake oil; your mileage may vary. But then again when using proprietary extensions BT version does not really matter either.