single image lightbox problem

Well it will be best to see the page it concerns.
If the other images have no link to the lightbox – they are not in the gallery – see here – only the one on the left bottom corner has lightbox link.
https://webers-testseite.de/images-with-no-link/

but if you have more links to lightbox – you only can state lightbox groups for the lightbox gallery.
( or put them in different color-sections will do the jop too)
you can put this to your child-theme functions.php:
( pay attention the code is site specific set to page ID: 123456 – you had to input your page-id on that )

// group images for lightbox galleries with custom-class:  (group-1, group-2 etc)
function handle_lightbox_groups_in_gallery(){
if(is_page(123456)){
?>
<script>
(function($){
    $(window).load(function() {
        $('.lightbox-added').each(function(index, element){
          $('.group-'+index+' .lightbox-added').attr('data-group', index);
        });
        var groups = {};
        $('.lightbox-added').each(function() {
          var id = parseInt($(this).attr('data-group'), 10);
          if(!groups[id]) {
            groups[id] = [];
          }
          groups[id].push( this );
        });
        $.each(groups, function() {
          $(this).magnificPopup({
              type: 'image',
              mainClass: 'avia-popup mfp-zoom-in mfp-image-loaded',
              closeOnContentClick: false,
              closeBtnInside: false,
              gallery: { enabled:true }
          })
        });
    });
})(jQuery);
</script>
<?php
}
}
add_action('wp_footer', 'handle_lightbox_groups_in_gallery');

to prepare your groups then – you have to give to each image alb element the custom-Class: group-1, group-2 etc.

see here – allthough these images are in the same color-section – they open in two seperated galleries.:
https://webers-testseite.de/lightbox-grouping/