if you click Masonry Gallery it should Open in New Tab

The thing is that these elements are created after the DOM has been loaded. So we had to have a “Listener” when there was a Change on the DOM.
I knew that former function – and i know that this is a deprected one- but the function still seems to works in all browsers

Maybe someone ( more competent than I am ) could tell us to do it with MutationObserver – This seems to be the modern variant of listening to DOM.

function masonry_newtab(){
?>
<script>
(function($){
  $(document).ready(function($){ 
    $('body').on('DOMNodeInserted', function(){
      $('.av-masonry-container a').attr('target', '_blank');
    });
  });
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'masonry_newtab');

try this in functions.php of your child-theme instead the old rule

This function was simply ingenious, unfortunately this is a thing of the past and will probably no longer be supported properly in future browsers.
The whole thing has probably been replaced by the MutationObserver method. I haven’t done it yet – at least not in a way that I know how to use it in combination with e.g. jQuery to insert the href attribute.
Maybe one of the developers could give us some advice.