Entity autocomplete fails to create terms if term exists in other vocabulary

Comparing validateEntityAutocomplete in Drupal\Core\Entity\Element\EntityAutocomplete and Drupal\selectize\Element\SelectizeEntityAutocomplete it seems the issue is the options are set differently now :

$options = $element['#selection_settings'] + [
  'target_type' => $element['#target_type'],
  'handler' => $element['#selection_handler'],
];

Drupal\Core\Entity\Element\EntityAutocomplete

$options = [
  'target_type' => $element['#target_type'],
  'handler' => $element['#selection_handler'],
  'handler_settings' => $element['#selection_settings'],
];

Drupal\selectize\Element\SelectizeEntityAutocomplete

Changing it the selectize one to match seems to solve the issue.

I've made a patch which addresses this issue and the drag and drop issue in https://www.drupal.org/project/selectize/issues/3458652 📌 Drag and drop no longer works Active for testing.