I wonder if this is related to @ian’s post Strange Emoncms MQTT publish phenomena (not solved but the reasons understood see https://community.openenergymonitor.org/t/mqtt-log-entries/7784). Looking again at the code, perhaps the process model can reuse the existing mqtt connection started by the phpmqtt_input script or it could start a single publishing client if connect was called in the _construct rather than the publish_to_mqtt method itself:
I will raise as an issue.