ROS 2 message_filters TimeSynchronizer minimal example does not reach callback function

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Description

I am trying to get the message_filters::TimeSynchronizer to work on my Linux Ubuntu 18.04 (Linux version 5.4.0-54-generic) and wrote a minimal node, according to other entries in this forum and the ros2 tutorials. This node simply publishes two Temperature messages with the exact same timestamp and then subscribes to these topics to an instance of message_filters::TimeSynchronizer. Unfortunately, the Synchronization does not work as expected and at the same time does not output any Warnings, Errors or similar, that would help debugging. The issue is the following:

When testing the plain message_filters::Subscriber instances for the two topics, I could verify, that the messages actually do come in and have the exact same timestamp. But still after adding them to the TimeSynchonizer instance, the according callback function is never called somehow. I also tested it with a Policy (ApproximateTime), but also there, the callback was never called. I even made sure, after reading through similar issues in here, to use the same QoS in both the publishers and subscribers without any success...

Platform: Tested both in Ubuntu 20.04 with Foxy, as well as in Ubuntu 18.04 with Eloquent with the same outcome.

Code: The test node I wrote for this to reproduce the error looks the following:

Package XML

<?xml version="1.0"?>
<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema" ?>
<package format="3">
  <name>temp_sync</name>
  <version>0.0.0</version>
  <description>TODO: Package description</description>
  <maintainer email="tobias@mantistechnologies.ch">tobias</maintainer>
  <license>TODO: License declaration</license>

  <depend>sensor_msgs</depend>
  <depend>message_filters</depend>
  <depend>rclcpp</depend>

  <buildtool_depend>ament_cmake</buildtool_depend>

  <test_depend>ament_lint_auto</test_depend>
  <test_depend>ament_lint_common</test_depend>

  <export>
    <build_type>ament_cmake</build_type>
  </export>
</package>

CmakeLists.txt

cmake_minimum_required(VERSION 3.5)
project(temp_sync)

# Default to C99
if(NOT CMAKE_C_STANDARD)
  set(CMAKE_C_STANDARD 99)
endif()

# Default to C++14
if(NOT CMAKE_CXX_STANDARD)
  set(CMAKE_CXX_STANDARD 14)
endif()

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic)
endif()

# find dependencies
find_package(ament_cmake REQUIRED)
find_package(sensor_msgs REQUIRED)
find_package(rclcpp REQUIRED)
find_package(message_filters REQUIRED)
# uncomment the following section in order to fill in
# further dependencies manually.
# find_package(<dependency> REQUIRED)

add_executable(syncer src/temp_sync.cpp)
ament_target_dependencies(syncer rclcpp sensor_msgs message_filters)

if(BUILD_TESTING)
  find_package(ament_lint_auto REQUIRED)
  # the following line skips the linter which checks for copyrights
  # uncomment the line when a copyright and license is not present in all source files
  #set(ament_cmake_copyright_FOUND TRUE)
  # the following line skips cpplint (only works in a git repo)
  # uncomment the line when this package is not in a git repo
  #set(ament_cmake_cpplint_FOUND TRUE)
  ament_lint_auto_find_test_dependencies()
endif()

install(TARGETS syncer
  EXPORT export_${PROJECT_NAME}
  DESTINATION lib/${PROJECT_NAME})

ament_package()

Cpp Node

#include <message_filters/subscriber.h>
#include <message_filters/time_synchronizer.h>

#include <chrono>
#include <rclcpp/rclcpp.hpp>

#include "sensor_msgs/msg/temperature.hpp"

using namespace std::chrono_literals;

class SyncerNode : public rclcpp::Node {
 public:
  SyncerNode() : Node("syncer") {
    rclcpp::QoS qos(10);
    auto rmw_qos_profile = qos.get_rmw_qos_profile();

    publisher_temp1_ =
        this->create_publisher<sensor_msgs::msg::Temperature>("temp_1", qos);
    publisher_temp2_ =
        this->create_publisher<sensor_msgs::msg::Temperature>("temp_2", qos);

    timer_ = this->create_wall_timer(
        500ms, std::bind(&SyncerNode::TimerCallback, this));

    subscriber_temp1_.subscribe(this, "temp_1 ...