The problem

I copied the arduino integration into custom_components:

cp -R /srv/homeassistant/lib/python3.7/site-packages/homeassistant/components/arduino/ /home/homeassistant/.homeassistant/custom_components/

and made homeassistant the owner:

sudo chown -R homeassistant:homeassistant arduino/

Environment

Latest hass, latest raspbian on Pi 4.

• Home Assistant Core release with the issue: 108.9
• Integration causing this issue: arduino
• Link to integration documentation on our website: https://www.home-assistant.io/integrations/arduino/

Problem-relevant configuration.yaml

arduino:
  port: /dev/ttyACM0

sensor:
  - platform: arduino
    pins:
      4:
        name: Door Ring

switch:
  - platform: arduino
    pins:
      21:
        name: Bell Speaker
        initial: true
      5:
        name: Door Opener

Traceback/Error logs

2020-04-28 21:41:30 WARNING (SyncWorker_4) [custom_components.arduino] The version of the StandardFirmata sketch was notdetected. This may lead to side effects
2020-04-28 21:41:30 ERROR (SyncWorker_11) [custom_components.arduino.switch] A connection has not been made to the Arduino board
2020-04-28 21:41:30 ERROR (SyncWorker_14) [custom_components.arduino.sensor] A connection has not been made to the Arduino board

Additional information

So the integration is working as expected outside of the custom_components, but if I want to improve this integration I basically have to use the custom_components dir for development purposes.
Something of the setup procedure of custom integrations is breaking the arduino one.