Missing “header-scrolled” class under mobile

you have to have a child-theme avia-snippet-sticky-header.js file.
on line 79 there is:

remove that && ! isMobile so there is only :

if( shrinking )

if you like to minify your theme script and css files – put both files ( the script itself and the minified script to a js subfolder to your child-theme. The edited script : you can minfy the code at: https://www.toptal.com/developers/javascript-minifier

This comes to your child-theme functions.php:

function wp_change_sticky_header_script() {
  wp_deregister_script( 'avia-sticky-header' );

  $vn = avia_get_theme_version();
  $options = avia_get_option();
  $child_theme_url = get_stylesheet_directory_uri();
  $min_js = avia_minify_extension( 'js' );

  $condition  = ( isset( $options['header_position'] ) && $options['header_position'] == 'header_top' );
  $condition2 = ( isset( $options['header_sticky'] ) && $options['header_sticky'] == 'header_sticky' ) && $condition;
  avia_enqueue_script_conditionally( $condition2 , 'avia-sticky-header-child', "{$child_theme_url}/js/avia-snippet-sticky-header{$min_js}.js", array('avia-default'), $vn, true);
}
add_action( 'wp_enqueue_scripts', 'wp_change_sticky_header_script', 100 );