SDTT error "URL must point to the same page" and formatting issues with JSON-LD for "ItemList"

Question

I'm trying to implement JSON-LD markup on my current WordPress project and ran into what I understand is a fairly common problem: SDTT responds with a "URL must point to the same page" error.

Following the recommendations from this page, I modified my code but the error still pops-up.

        $mup =  '{'."\n";
        $mup .= '           "@type":"ListItem",'."\n";
        $mup .= '           "position":'. ($i-1) .','."\n";
        $mup .= '           "item": {'."\n";
        $mup .= '               "@type" : "Report",'."\n";
        $mup .= '               "author":"'. get_bloginfo('name') .'",'."\n";
        $mup .= '               "publisher":{'."\n";
        $mup .= '                   "@type": "Organization",'."\n";
        $mup .= '                   "name": "'. get_bloginfo('name') .'",'."\n";
        $mup .= '                   "logo": "'. $image[0]. '"'."\n";
        $mup .= '               },'."\n";
        $mup .= '               "datePublished": "'. get_the_date("Y-m-d") .'",'."\n";
        $mup .= '               "headline" : "'. get_the_title() .'",'."\n";
        $mup .= '               "description" : "'.get_the_excerpt().'",'."\n";
        $mup .= '               "genre" : "Case study",'."\n";
        $mup .= '               "url":"'. get_permalink() .'"'."\n";
        $mup .= '           }'."\n";
        $mup .= '       }';
        $markup[] = $mup;

And in the footer:

<script type="application/ld+json">
    {
        "@context": "http://schema.org",
        "@type": "ItemList",
        "itemListElement": [<?php echo (join(",\n", $markup))."\n" ?>]
    }
</script>

Example of generated code:

<script type="application/ld+json">
    {
        "@context": "http://schema.org",
        "@type": "ItemList",
        "itemListElement": [{
        "@type":"ListItem",
        "position":1,
        "item": {
            "@type" : "Report",
            "author":"RPA導入コンサルティング会社・RPAツール・RPAソフトを徹底比較",
            "publisher":{
                "@type": "Organization",
                "name": "RPA導入コンサルティング会社・RPAツール・RPAソフトを徹底比較",
                "logo": "http://rpa-media.net/wp-content/uploads/2018/04/logo.png"
            },
            "datePublished": "2018-04-04",
            "headline" : "事例１",
            "description" : "",
            "genre" : "Case Study",
            "url":"http://rpa-media.net/case-study/%e4%ba%8b%e4%be%8b%ef%bc%91/"
        }
    }
]
        }
    </script>

And beautified code:

<script type="application/ld+json">
  {
    "@context": "http://schema.org",
    "@type": "ItemList",
    "itemListElement": [{
      "@type": "ListItem",
      "position": 1,
      "item": {
        "@type": "Report",
        "author": "RPA導入コンサルティング会社・RPAツール・RPAソフトを徹底比較",
        "publisher": {
          "@type": "Organization",
          "name": "RPA導入コンサルティング会社・RPAツール・RPAソフトを徹底比較",
          "logo": "http://rpa-media.net/wp-content/uploads/2018/04/logo.png"
        },
        "datePublished": "2018-04-04",
        "headline": "事例１",
        "description": "",
        "genre": "Case Study",
        "url": "http://rpa-media.net/case-study/%e4%ba%8b%e4%be%8b%ef%bc%91/"
      }
    }]
  }
</script>

This code generates the error BUT, if I take the generated code, beautify it in Atom then use it in SDTT, the error disappears.

If I try to use the beautified generated code directly on the page (as is, no PHP), the error comes back.

Any way I could implement that properly? Is my data badly structured? Or is it the way I am outputting it?