I’m using this code to add the alt text to my featured images on upload.

$image_title = get_post( $post->ID )->post_title;
// Sanitize the title:  remove hyphens, underscores & extra spaces:
$image_title = preg_replace( '%s*[-_s]+s*%', ' ',  $image_title );
// Sanitize the title:  capitalize first letter of every word (other letters lower case):
$image_title = ucwords( strtolower( $image_title ) );
// Set the image Alt-Text
add_post_meta( $post->ID, '_wp_attachment_image_alt', $image_title );

It’s working and I can see it on it’s database table wp_postmeta under it’s post ID.

Altough it’s correctly registred on the database, the featured image doesn’t have the Alt text (see the image bellow)

What could be the problem?