Deal with fields having string value in export yml in stead of array

We have custom entity type X, which has a definition in src/Entity/X.php

public static function baseFieldDefinitions(EntityTypeInterface $entity_type): array {
  $fields['title']
    ->setSettings([
      'text_processing' => 0,
    ])
    ->setTranslatable(TRUE);

In our node type, we have entity reference to these X.

The export exports 'title' like this:

field_x:
    -
      uuid: 4ecb6039-3fd7-4040-8bc3-0be4d3d765bb
      entity_type: x
      bundle: bundle_x
      base_fields:
        title: 'The title of the X'

When importing this, the importer fails in importFieldValue, because it expects the 'title' to be an array value, not a string.

It seems the code it tries to treat entity 'property' (base field) as a UI field, which always has value as an array.