Plugin handlers don't load properly

The discovery of the plugin handlers doesn't seem to work properly. I wanted to get at all conditional_field plugin handlers in use, but I only get back the 4 plugin handlers which are added through StaticDiscovery.

Source code showing the problem is attached:
- Put the attached ConditionalFieldsMissingDefinitions.txt file in the src directory of conditional_fields and change the extension from txt to php. It doesn't have real functionality, it's just showing the problem.
- In ConditionalFieldsFormHelper.php change the start of function getState() to look like this:

  protected static function getState(EntityFormDisplayInterface $form_display, $dependee, $dependee_form_field, $options, $form_state = NULL) {
    $plugin_cache = ConditionalFieldsMissingDefinitions::getInstance();

- Add content in Drupal using a form that uses ConditionalFields.
- Look at the "Recent Log Messages" in Drupal showing the output. In the output you will see only 4 entries, while I would have expected at least all of the handlers in the "conditional_fields\src\Plugin\conditional_fields\handler" directory to be present.

I'm using Drupal 8.8.2