Thumbnail image disappears from post/page after updating the original text file

A weird behaviour happens with large thumbnails for posts and pages.

Take this test post for example.

Here is the metadata included in the file txt file :

Date: 26/05/2024
Summary: Du travail d'escroc
Thumbnail: https://res.cloudinary.com/poirpom/image/upload/v1717446275/Tests/240603-9.jpg
Slug: /encore-un-blog-post

and here is the HTML code from the entry.html template :

  <div style="min-height:6rem;flex-direction:row;align-items:center;" class="flex mt3 pt3 ">
    <div class="w-100" style="margin-bottom: 2rem !important;">
<!-- Code supprimé et stocké dans FSnotes -->
        <div class="w-80"> <!-- suppression du pr4 -->
          <div class="hero">
            {{{titleTag}}}
            <img src="{{{thumbnail.large.url}}}" style="min-width: 100%;min-height: 2px;background-color: #1D191E;margin-top: 20px;" />
            <div class="summary">{{{summary}}}
            </div>
          </div>  
          <div class="lh-copy" style="margin-top: 10px !important; text-align: right !important;"> <!-- bloc date et tags déplacés -->
 
    {{^page}}
  {{#date}}<a href="{{url}}" class="black-30 f5 no-underline"><span>{{#formatDate}}DD.MM.YYYY{{/formatDate}}</span></a>{{/date}} • 
    <span class="black-50 f5">{{#tags}}<a href="/tagged/{{slug}}" class="no-underline">{{tag}}</a>{{^last}}, {{/last}}{{/tags}}</span>
<!-- <hr class="intermediaire" /> -->
{{/page}}
      </div> 
      </div>
    </div>
      
    </div>

I published the post — the thumbnail appeared just fine.

if, for some reason, I update the text file of the post, save it, wait for blot to do its thing, then the thumbnail doesn't appear anymore on the page.

Same happen with any blog post or any page. Each time I edit the content, thumbnail doesn't show up anymore.

When I look at the inspector, src appears in the html code, but without any url (screenshot here).

The only way i found to make the thumbnail image appear again is to regenerate all thumbnails through the reset your folder settings. (Which is no big deal, but kind of annoying).

Is there something I am missing or doing wrong here ?

Thanks in advance for any advice.