Add New Items to Taxonomy creates inconsistent results

My plugin includes advanced select boxes for two taxonomies, each of which is configured to enable the 'add_new' function. However, the 'Add New' button only appears for one of the two. The function works properly when I use it via builder, but when I transfer it to php code, it starts erroring.

The code:

         array (
                'id' => $prefix . 'author',
                'type' => 'taxonomy',
                'name' => esc_html__( 'Author', 'pub-fields' ),
                'placeholder' => esc_html__( 'Select an author', 'pub-fields' ),
                'taxonomy' => 'author',
                'field_type' => 'select_advanced',
                'multiple' => true,
                'columns' => 6,
                'add_new' => true,
            ),
            array (
                'id' => $prefix . 'org',
                'type' => 'taxonomy',
                'name' => esc_html__( 'Publication Organisation', 'pub-fields' ),
                'placeholder' => esc_html__( 'Select an organisation', 'pub-fields' ),
                'taxonomy' => 'organisation',
                'field_type' => 'select_advanced',
                'multiple' => true,
                'columns' => 6,
                'add_new' => true,
            ),

Image of the result: