TYPO3 Core

With following patch you have added the title field additionally to __label key in $theData:

https://review.typo3.org/#/c/41622/4

After that the thumbnails will be rendered, but only for array key "title". Later in in addElement there is only a check against __label:

if ($colType === 'td' && array_key_exists('__label', $data)) {
    $fields[0] = '__label';
}

As long as "__label" was not filled with any thumbnails, they will not be displayed.

Stefan