Number field not showing std

I have a number field where I've entered a value via 'std', but it does not show in the metabox. Any ideas?

Here's the code I used...

					array(
						'name' => esc_html__( 'Headline Font Size', 'textdomain' ),
						'id'   => $prefix.'headline_font_size',
						'tab'  => 'hero_text',
						'type' => 'number',
						'min'  => 16,
						'max'  => 100,
						'step' => 2,
						'std'  => 48,
						'visible' => array($prefix . 'hero_type', '=', 'headline_sub')
					),