Migration Update "Integrity constraint violation"

We created a custom destination plugin to lookup the value in the getEntity() method, the only suspicious part is I hardcoded english because we aren't migrating the langcode:

<?php

declare(strict_types=1);

namespace Drupal\custom_migration\Plugin\migrate\destination;

use Drupal\migrate\Attribute\MigrateDestination;
use Drupal\migrate\Plugin\migrate\destination\EntityContentBase;
use Drupal\migrate\Row;

/**
 * The 'registration_settings' destination plugin.
 */
#[MigrateDestination(
  id: 'registration_settings'
)]
final class RegistrationSettings extends EntityContentBase {

  /**
   * The entity type plugin id for RegistrationSettings entity.
   */
  protected static function getEntityTypeId($plugin_id): string {
    return 'registration_settings';
  }

  /**
   * {@inheritdoc}
   */
  protected function getEntity(Row $row, array $old_destination_id_values) {
    // Extract the keys to determine if the entity already exists.
    $entity_type_id = $row->getSourceProperty('entity_type');
    $entity_id = $row->getSourceProperty('entity_id');
    // Hardcode en if not available.
    $langcode = $row->getSourceProperty('langcode') ?? 'en';

    // Load the existing entity if it exists.
    $existing_entities = $this->storage->loadByProperties([
      'entity_type_id' => $entity_type_id,
      'entity_id' => $entity_id,
      'langcode' => $langcode,
    ]);

    // Get the first match, if any.
    $entity = reset($existing_entities);

    if ($entity) {
      // Allow updateEntity() to change the entity.
      $entity = $this->updateEntity($entity, $row) ?: $entity;
    }
    else {
      // Attempt to ensure we always have a bundle.
      if ($bundle = $this->getBundle($row)) {
        $row->setDestinationProperty($this->getKey('bundle'), $bundle);
      }

      // Stubs might need some required fields filled in.
      if ($row->isStub()) {
        $this->processStubRow($row);
      }
      $entity = $this->storage->create($row->getDestination());
      $entity->enforceIsNew();
    }

    return $entity;
  }
}

@joelpittet thanks for posting this, your destination plugin looks good, glad you found a solution. However, I was not able to reproduce the error in my test environment - do you have multiple languages installed on your site by chance.

Yes and no:), we have language enabled because search_api or the solr extension requires it but we are doing that one to one, und to en.

Do you think that is the ticket?

Could be - I'll reach out on Slack