InvalidArgumentException: "data-region" is an invalid render array key. Value should be an array but got a string. in Drupal\Core\Render\Element::children() (line 98 of core/lib/Drupal/Core/Render/Element.php).

Update: This is super strange!

This is the full code:

$build['regions'][$region_name] = [
          '#attributes' => [
            'class' => [
              'js-lpb-region',
            ],
            'data-region' => $region_name,
            'data-region-uuid' => $entity->uuid() . '-' . $region_name,
            'data-lpb-ui-id' => $entity->uuid() . '-' . $region_name,
          ],
        ];

So I don't think that will affect anything, bein inside #attributes!

Sorry, this seems to be an issue with https://www.drupal.org/project/layout_section_classes →

Final update: This was a mistake in a custom layout twig file.
We had interated through a render array with #attributes and the expected sub-render array and rendered them individually. That caused the issue.