Drupal\Component\Plugin\Exception\PluginNotFoundException: The "field:synonyms.behavior.autocomplete_taxonomy_term_content_category_field_synonyms" plugin does not exist

I have traced the problem to Drupal\synonyms\SynonymsService\ProviderService

/**
   * Get a list of enabled synonym providers.
   *
   * @param string $entity_type
   *   Entity type for which to conduct the search.
   * @param string|array $bundle
   *   Single bundle or an array of them for which to conduct the search. If
   *   null is given, then no restrictions are applied on bundle level.
   *
   * @return \Drupal\synonyms\Entity\Synonym[]
   *   The array of enabled synonym providers
   */
  public function getSynonymConfigEntities($entity_type, $bundle) {
    $entities = [];

    if (is_scalar($bundle) && !is_null($bundle)) {
      $bundle = [$bundle];
    }

    foreach ($this->entityTypeManager->getStorage('synonym')->loadMultiple() as $synonym_config) {

      $provider_instance = $synonym_config->getProviderPluginInstance();
      $provider_definition = $provider_instance->getPluginDefinition();
      if ($provider_definition['controlled_entity_type'] == $entity_type && (!is_array($bundle) || in_array($provider_definition['controlled_bundle'], $bundle))) {
        $entities[] = $synonym_config;
      }
    }

    return $entities;
  }

I am closing this issue. It seems this error has gone away after downgrading the module to 8.x version and then upgrading it. This could have been due to a faulty installation or a faulty upgrade.