The test creates mock_transport = Mock(spec=AsyncHttpTransport) and sets it as mock_client.transport. However, in the actual _flush() function (asyncio.py line 71), there's a check isinstance(client.transport, AsyncHttpTransport) which will always return False for a Mock object, even with spec=AsyncHttpTransport. The spec parameter only controls which attributes are available on the mock, not its type hierarchy. As a result, _flush() returns early without calling close_async(), causing the assertion mock_client.close_async.assert_called_once() to fail.

The test test_async_transport_concurrent_requests sets sentry_sdk.get_global_scope().set_client(client) at line 998 but never cleans it up with a finalizer. Other similar tests in this file (e.g., test_transport_works_async at line 928, test_async_transport_rate_limiting_with_concurrency at line 1024) properly add request.addfinalizer(lambda: sentry_sdk.get_global_scope().set_client(None)). This inconsistency can cause test pollution where the client leaks into subsequent tests.