Grav - why page.media[image.thumbnail].url works and page.media[image.flag].url doesn't, even if defined

Question

I am new to Grav, but I was attracted by its Twig simplicity.

I have the following newbie question: I imported a theme "Multiverse", that I want to change slightly. Why "page.media[image.thumbnail].url" works and "page.media[image.flag].url" returns blank, even if defined in YAML?

Multiverse blueprint.yaml in Admin:

title: Gallery
'@extends':
    type: default

form:
  fields:
    tabs:
      type: tabs
      active: 1
      fields:
        images:
          type: tab
          title: Images
          fields:
            header.images:
              name: images
              type: list
              label: Images
              fields:
                .title:
                  type: text
                  label: Title
                  help: Image's title.
                .thumbnail:
                  type: text
                  label: Thumbnail
                  help: Thumbnail's filename.
                .description:
                  type: text
                  label: Description
                  help: Image's description.
                .flag:
                  type: text
                  label: Flag
                  help: Flag.
                .keywords:
                  type: text
                  label: Keywords
                  help: Keywords.

Then, in home.twig of Multiverse, I have:

        {% for image in page.header.images %}
        <article class="thumb">
            <a href="{{ page.media[image.thumbnail].url }}" class="image">
                <img src="{{ page.media[image.thumbnail].url }}" alt="{{ image.title|escape }}" />
            </a>
            <h2>{{ image.title|raw }}</h2>
            <p>{{ image.description|raw }}</p>
            <img src="{{ page.media[image.flag].url }}" alt="{{ image.keywords|escape }}" />
        </article>
        {% endfor %}

So, I only added this line:

        <img src="{{ page.media[image.flag].url }}" alt="{{ image.keywords|escape }}" />

I added values for all fields in Admin Expert mode in Images tab:

        title: 'Armenia (Asia)'
        thumbnail: khor-virap-1781518.jpg
        description: 'Țara muntelui Ararat, unde se zice că s-a oprit arca lui Noe și cu prima biserică creștină fondată în secolul I.'
        flag: ar.png
        color: blue
        keywords: Europe
        ribbon: LUX
        stars: '2'

As you can see, both "thumbnail" and "flag" are defined in blueprint and in media. Both images are uploaded in the folder:

thumbnail: khor-virap-1781518.jpg
flag: ar.png

Why page.media[image.thumbnail].url returns the url correctly: /user/pages/01.home/khor-virap-1781518.jpg, but page.media[image.flag].url returns blank?

Is "thumbnail" some kind of special field? I see it defined as text:

                .thumbnail:
                  type: text
                  label: Thumbnail
                  help: Thumbnail's filename.

Thank you!

What is confusing is that all fields that I added, like keywords, ribbon, even flag, work if I output them: {{ image.flag }} works {{ image.flag.url }} doesn't work {{ page.media[image.flag].url }} doesn't work