We believe this is not a Unity issue, as we managed to crash the project by just clicking "Join Channel".
We created a test project playing audio looped back from a microphone and repeatedly disconnected/reconnected non-Apple bluetooth headsets to identify if the Unity audio engine reset was causing the issue and it's not the case.
We suspect that Agora's internal code might be interacting with the OS beyond the scope of Unity Engine, as the callstack provided doesn't point at any unity audio symbols.
If you can provide us with a repro project that doesn't use any Agora specific code, we can reopen the issue and investigate.