Why a reliable subscriber misses a latched topic

A ROS2 publisher advertises a low-rate "map" topic with `reliability=RELIABLE` and `durability=TRANSIENT_LOCAL` so that late-joining subscribers still receive the last map. A new subscriber starts up with a stricter-than-publisher QoS profile and never receives any message, even though `ros2 topic list` shows the topic. Explain what is going wrong in terms of DDS/ROS2 QoS compatibility, which specific policies (reliability, durability) can be incompatible, the general rule that governs whether a publisher and subscriber will connect, and how you would fix it. Also note one reason TRANSIENT_LOCAL durability matters for a topic like this.