As mentioned elsewhere on this forum, bluetooth audio actually has two separate devices. But you only hear one of them at a time. The other becomes a black hole.
There's a device for "bluetooth speakers", which gives you high quality playback with no microphone.
There's a device for "bluetooth headset", which gives you a mic with lower quality speakers.
When you ask for a mic, it switches the entire physical device to "bluetooth headset" mode, which then ignores what is sent to the "bluetooth speaker".