def setup():
  global kb, mqtt_client ......
==========BAD CODE HERE================
  mqtt_client.subscribe('/cardputer', mqtt__cardputer_event, qos=0)
=====================================

  M5.begin()
  kb = MatrixKeyboard()
  kb.set_callback(kb_pressed_event)
  mqtt_client = MQTTClient('cardputer', 'xxx.xxx.xxx.xxx', port=1883, user='xxx', password='xxx', keepalive=0)
  mqtt_client.reconnect()

.....

Note that the subscribe code generated Even Before M5.begin()!

You have to fix this BLOCKING Issue asap to make the block coding mqtt code useful. I can now only manually modify code to make sure the subscribe action works.