Subscriber only receives data from connected publisher after 'ros2 topic echo <name_of_topic>'

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Environment:

• ROS Galactic, built from source using instructions similar to https://docs.ros.org/en/galactic/Inst... (stopped after the 'Install dependencies using rosdep' sections, i.e. did NOT install additional DDS implementations)
• Lubuntu 20.04
• Additional repos of interest, also built from source:
  • https://github.com/ros-perception/ima..., 'ros2' branch
  • https://github.com/klintan/ros2_usb_c..., 'foxy-devel' branch

I have a couple of nodes, one of which publishes sensor_msgs::msg::Image data on a topic /smart_home/scc_image, the other subscribes to the data on this topic.

The publisher node is started with a custom launch file, which instantiates a node from the aforementioned ros2_usb_camera package (the topic remapping moves 'image' to 'scc_image', and 'sh_common_py' is a custom module):

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, LogInfo, Shutdown
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node
from ament_index_python.packages import get_package_share_directory
from os.path import join as ojoin

import sh_common_py

def generate_launch_description():
    cfg = ojoin(
        get_package_share_directory("sh_scc_image_stream"),
        "config",
        "params.yaml"
    )
    desc = LaunchDescription()
    desc.add_action(DeclareLaunchArgument(
        "camera_calibration_file",
        default_value=""
    ))
    desc.add_action(DeclareLaunchArgument(
        "camera_id",
        default_value="0"
    ))
    desc.add_action(Node(
        package="usb_camera_driver",
        executable="usb_camera_driver_node",
        name="sh_scc_image_stream",
        namespace="/smart_home",
        remappings=[("image", sh_common_py.topics.SCC_CAMERA_IMAGE)],
        parameters=[
            cfg,
            {"camera_calibration_file": LaunchConfiguration("camera_calibration_file")},
            {"camera_id": LaunchConfiguration("camera_id")},
        ],
        on_exit=Shutdown(),
    ))
    desc.add_action(LogInfo(msg=LaunchConfiguration("camera_calibration_file")))
    desc.add_action(LogInfo(msg=LaunchConfiguration("camera_id")))
    return desc

And this references a simple configuration file:

/smart_home:
    sh_scc_image_stream:
        ros__parameters: 
            frame_id: "sh_scc_image_stream_camera"
            image_width: 3840
            image_height: 2160
            fps: 30.0
            camera_id: 0

The subscriber node is from a custom package. The header declares the subscription as:

image_transport::Subscriber cap_image_raw_sub;

And is initialized in the source as:

cap_image_raw_sub = image_transport::create_subscription(
    this,
    ::sh::names::topics::SCC_CAMERA_IMAGE,
    std::bind(
        &sh::ColorPeaksCalculatorNode::cap_image_raw_callback,
        this,
        std::placeholders::_1
    ),
    "raw",
    rmw_qos_profile_sensor_data
);

Where sh::ColorPeaksCalculatorNode::cap_image_raw_callback has the following signature:

void sh::ColorPeaksCalculatorNode::cap_image_raw_callback(const sensor_msgs::msg::Image::ConstSharedPtr& msg);

Running the full system, I can see that these two nodes connect to the same topic:

$ ros2 topic info /smart_home/scc_image --verbose
Type: sensor_msgs/msg/Image

Publisher count: 1

Node name: sh_scc_image_stream
Node namespace: /smart_home
Topic type: sensor_msgs/msg/Image
Endpoint type: PUBLISHER
GID: 56.8b.10.01.47.47.5d.07.f0.e5.e7.23.00.00.15.03.00.00.00.00.00.00.00.00
QoS profile:
  Reliability: BEST_EFFORT
  Durability: VOLATILE
  Lifespan: 9223372036854775807 nanoseconds
  Deadline: 9223372036854775807 nanoseconds
  Liveliness: AUTOMATIC
  Liveliness lease duration: 9223372036854775807 nanoseconds

Subscription count: 1

Node name: sh_color_peaks_calculator
Node namespace: /smart_home
Topic type: sensor_msgs/msg/Image
Endpoint type: SUBSCRIPTION
GID: 66.4f.10.01.3d.3e.6d.25.6a.9f.f5.fd.00.00.1a.04.00.00.00.00.00.00.00.00
QoS profile:
  Reliability: BEST_EFFORT
  Durability: VOLATILE
  Lifespan: 9223372036854775807 nanoseconds
  Deadline: 9223372036854775807 nanoseconds
  Liveliness: AUTOMATIC
  Liveliness lease duration: 9223372036854775807 nanoseconds

However, the subscriber node is not behaving like it is receiving data. But as soon as I run ros2 topic echo /smart_home/scc_image, both this and the subscriber node start receiving data. When I terminate the echo command, the subscriber node continues to receive data as expected. Output I receive with ros2 topic echo /smart_home ... (more)