the preview displays the content of the template that renders the block. In your template, the first element displayed is the image. So the preview shows the image but in realty that’s not the image which is in your attribute “gutenberg_preview”. It’s just the image in your template.
to verify my theory, could you remove the url from your image and see if it changes anything.
Like that :
'example' => array(
'attributes' => array(
'mode' => 'preview',
'data' => array(
'gutenberg_preview' => '',
),
)
)
I think that your preview will not change
But if your remove the image in your template, your preview will change
