get_theme_mod doesn't return the theme customizer preview's new values in after_setup_theme hook

If I print the value of get_theme_mod( 'enable_sleek_header', false ) it is always the previously saved value. If I print the same thing in the header of the theme, it returns the value from customizer.

• Is it the expected behaviour?
• Am I using the wrong hook?

Thanks

?php

add_action( 'after_setup_theme', 'pagespeed_register_menus' );


    function pagespeed_register_menus() {

        //Not getting the modified theme_mod from the customizer without saving.
        register_nav_menus( array(
            'secondary' = __( 'Navigation above header', 'page-speed' ),
        ) );

        if ( get_theme_mod( 'enable_sleek_header', false ) ) {
            register_nav_menus( array(
                'header' = __( 'Navigation menu in header', 'page-speed' ),
            ) );
        } else {
            register_nav_menus( array(
                'primary' = __( 'Navigation below header', 'page-speed' ),
            ) );
        }
        register_nav_menus( array(
            'footer_links' = __( 'Footer links', 'page-speed' ),
        ) );

    }