I have same issue.
I tried to find out what is going on.
phone: Huawei P20 LITE
android: EMUI 9.1.0 (Android Ver 9)
headset: ANKER Soundcore Life U2
First, I connect the bluetooth headset which has BT profile "A2DP sink" and "HFP client".
(cf. A2DP profile is for audio output and HFP(Hands Free Profile) profile is for phone call/talk as headset with microphone.)
I opened BT device control panel of my android smartphone, to see the headset is connected with "media audio" and "call", In Japanese.
Open Teams app and join meeting, then no problem to use BT headset.
Open again BT device control panel, then open configuration (gear icon) of the headset.
Now I can see "Device profile" following three profiles: "talk audio", "media audio" and "sharing contacts"; each has a switch to enable/disable.
Now disable "talk audio", which seems HFP profile.
Then Teams cannot connect BT headset. In the meeting, I can see audio option including BT icon but cannot connect, soon fall back to device audio.
Still the BT headset is connected with "media audio", which seems A2DP profile, so every other application can use the headset as audio output.
Now I concluded that MS teams application only uses BT headset with microphone, HFP profile, but not BT speaker, A2DP profile.
MS Teams application on Windows has option to use any audio output, so one on Android should also have the option to use any audio output as well as headset, I think.