New alert component doesn't appear to support classic alerts.

I have the same issue. The option was first introduced at https://www.drupal.org/project/bootstrap_barrio/issues/3037643 → but in 5.x all messages include bootstrap_barrio:alert => alert.twig that wraps messages in <div class="alert-wrapper"></div> that is then referenced in alerts.scss that has:

.alert-wrapper {
  position: fixed;
  bottom: 0;
  left: 0;

A workaround is to apply

.alert-wrapper {
  position: unset !important;
}

see https://www.drupal.org/project/bootstrap_barrio/issues/3496694 🐛 Discrepancies between new alert.twig component and messages.js Active as well.

I think this is part of a larger issue where the template for alerts has deviated and there are now two versions -- one in new alert.twig component and one loaded via messages.js