responsive images attributes not working in theme override

When I look at the source of a Publii-generated page, I see

<figure class="featured-image">
  <img
     src="file:///Publii/sites/mysite/preview/media/posts/2/dressing.jpeg"
     srcset="[object Object]" sizes="[object Object]"
     height="2448"
     width="3264"
     alt="">
</figure>

Note the <code class=”EnlighterJSRAW” data-enlighter-language=”html”>sizes and <code class=”EnlighterJSRAW” data-enlighter-language=”html”>scrset attributes are <code class=”EnlighterJSRAW” data-enlighter-language=”html”>[object Object]. So something is wrong with my responsive images configuration. Is there a way to troubleshoot this issue? I assume the problem is in my config.json file. Here’s an excerpt of files.responsiveImages from that file:

{
    "name": "Blank",
    "etc" : "...",
    "files": {
        "responsiveImages": {
            "featuredImages": {
                "sizes": {
                    "post": "(max-width: 600px) 100vw, (max-width: 800px) 87vw, 700px",
                    "amp": "(max-width: 768px) 100vw, 768px"
                },
                "dimensions": {
                    "xs": {
                        "width": 300,
                        "height": "auto",
                        "group": "post,amp"
                    },
                    "sm": {
                        "width": 480,
                        "height": "auto",
                        "group": "post,amp"
                    },
                    "md": {
                        "width": 768,
                        "height": "auto",
                        "group": "post,amp"
                    },
                    "lg": {
                        "width": 1024,
                        "height": "auto",
                        "group": "post"
                    },
                    "xl": {
                        "width": 1360,
                        "height": "auto",
                        "group": "post"
                    },
                    "2xl": {
                        "width": 1600,
                        "height": "auto",
                        "group": "post"
                    }
                }
            }
        }
}