Gravity topic is empty when using IMU sensor from Isaac Sim

I am using Isaac Sim with a drone model equipped with stereo cameras and an IMU sensor. Through Action Graph, I publish the stereo camera images, camera info, and IMU data to ROS 2, then use Isaac ROS VSLAM for localization.
To verify whether the IMU data is actually being used, I checked the gravity topic using
ros2 topic echo /visual_slam/vis/gravity
However, the topic outputs nothing. As I understand it, this topic should normally publish the estimated gravity vector (or initialized gravity) when IMU fusion is working.
Could you please advise how to solve this issue? Thank you!
My generate_launch_description function is configured as follows:

def generate_launch_description():
    visual_slam_node = ComposableNode(
        name='visual_slam_node',
        package='isaac_ros_visual_slam',
        plugin='nvidia::isaac_ros::visual_slam::VisualSlamNode',
        remappings=[
            ('visual_slam/image_0',        'front_stereo_camera/left/image_rect_color'),
            ('visual_slam/camera_info_0',  'front_stereo_camera/left/camera_info'),
            ('visual_slam/image_1',        'front_stereo_camera/right/image_rect_color'),
            ('visual_slam/camera_info_1',  'front_stereo_camera/right/camera_info'),
            ('visual_slam/imu',            '/imu'),
        ],
        parameters=[{
            "verbosity": 2,
            'use_sim_time': True,
            'num_cameras': 2,
            'rectified_images': True,
            'enable_image_denoising': True,
            'enable_imu_fusion': True,
            'gravitational_force': [0.0, 0.0, -9.81],
            'imu_frame': 'imu',
            'gyro_noise_density': 0.000244,
            'gyro_random_walk': 0.000019393,
            'accel_noise_density': 0.001862,
            'accel_random_walk': 0.003,
            'map_frame': 'map',
            'odom_frame': 'odom',
            'base_frame': 'base_link',
            'publish_map_to_odom_tf': True,
            'publish_odom_to_base_tf': True,
            'enable_slam_visualization': True,
            'enable_observations_view': True,
            'enable_landmarks_view': True,
            'image_jitter_threshold_ms': 33.33,
        }])
    visual_slam_launch_container = ComposableNodeContainer(
        name='visual_slam_launch_container',
        namespace='',
        package='rclcpp_components',
        executable='component_container',
        composable_node_descriptions=[
            visual_slam_node,
        ],
        output='screen',
    )
    return launch.LaunchDescription([visual_slam_launch_container])