add the following code in functions.php file of monomyth theme to fix the hreflang urls issue for polylang plugin.

gistfile1.txt

add_filter( 'pll_rel_hreflang_attributes', function( $hreflangs ) {

   $app_slug=aw2_library::get('app.slug');

   if($app_slug == 'root') return $hreflangs;
    
    $default_lang = pll_default_language();
    $hreflangs_new = array(); 
    foreach($hreflangs as $key=>$url){
        $url_slug = site_url().'/'.$key;
        if($key == $default_lang){
            $url_slug = site_url();
        }
      
        $hreflangs_new[$key] = str_replace( $url_slug, $url_slug.'/'.$app_slug, $url );
    }

   //loop through the array and replace the 
    return $hreflangs_new;
} );