Custom Facet Processor Not Displaying

Problem/Motivation

Since the facet values return all lowercase values, we need to apply CSS to ensure the first letter is always capitalized. The problem is that the CSS will also capitalize words like And, Or, etc. As such we decided to create a custom module to create a processor that would allow us to replace values as needed. When we tried the following it does not show in the Facet settings.

Steps to reproduce:

1. Create a custom module, mymodule.
2. Enable module and clear cache
3. Create a view (myview) with a page display
4. Add Article's Category field to Search index (Article Category >> Taxonomy term >> Name)
5. Create facet (myfacet) where source is myview and field is Article Category >> Taxonomy term >> Name
6. Navigate to /admin/config/search/facets/myfacet/edit and observe the processor is not visible or found anywhere on the page.

Code in question

mymodule/src/Plugin/facets/processor/ReplaceProcessor.php

<?php
namespace Drupal\mymodule\Plugin\facets\processor;

use Drupal\Core\Form\FormStateInterface;
use Drupal\facets\FacetInterface;
use Drupal\facets\Processor\BuildProcessorInterface;
use Drupal\facets\Processor\ProcessorPluginBase;

/**
 * Provides a processor that replaces values.
 *
 * @FacetsProcessor(
 *   id = "replace",
 *   label = @Translation("Replace processor"),
 *   description = @Translation("Replace returned values."),
 *   stages = {
 *     "build" = 40
 *   }
 * )
 */
class ReplaceProcessor extends ProcessorPluginBase implements BuildProcessorInterface {
  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state, FacetInterface $facet) {
    $replacementValues = $this->getConfiguration()['replacements'];
    $build['replacements'] = [
      '#title' => $this->t('Replacement values.'),
      '#type' => 'textarea',
      '#default_value' => !is_null($config) ? $config->getConfiguration()['replacements'] : '',
      '#description' => $this->t("Used to replace values."),
    ];
    return $build;
  }
  /**
   * {@inheritdoc}
   */
  public function build(FacetInterface $facet, array $results) {
    $replacementValues = $this->getConfiguration()['replacements'];

    // Get the config and split by new line.
    $replacementResults = preg_split('/\r\n|\r|\n/', $replacementValues);
    $replacement_rows = [];

    // Add values into array.
    foreach($replacementResults as $row) {
      $replacementValue = explode('|', $row);
      $replacement_rows[$replacementValue[0]] = $replacementValue[1];
    }

    /** @var \Drupal\facets\Result\Result $result */
    foreach ($results as $result) {
      if(isset($replacement_rows[$result->getRawValue()])) {
        $result->setDisplayValue($replacement_rows[$result->getRawValue()]);
      }
    }

    return $results;
  }
}

mymodule/config/schema/mymodule.processor.schema.yml

plugin.plugin_configuration.facets_processor.*:
  type: config_object

plugin.plugin_configuration.facets_processor.replace:
  type: mapping
  label: 'Settings for ns facet processors.'
  mapping:
    replacements:
      type: text
      label: 'Replacement values'