Embedding in static page markdown file gives strange visuals

Hi there,
I am trying to embed my model from sketchfab in a Jekyll website, that I currently have not put online yet.
The file is a markdown file.

When inserting the embed code, I suddenly see lines where there are not supposed to be lines:

I tried to pinpoint the problem, and it seems to be happening only when pasting the “scr” link into the iframe. When exchanging the link to any other website, I don’t get a problem, but with it, I do…

Here us the markdown document:

title: "3D Visualisierung"
image: "/assets/images/background.jpg"
description: "Oberflächen and stuff"
---
This is the model:

    <div class="sketchfab-embed-wrapper">
        <iframe title="A 3D model" width="640" height="480" src="https://sketchfab.com/models/5fd6a228227743d09895a4640936a47c/embed?preload=1&amp;ui_controls=1&amp;ui_infos=1&amp;ui_inspector=1&amp;ui_stop=1&amp;ui_watermark=1&amp;ui_watermark_link=1" frameborder="0" allow="autoplay; fullscreen; vr" mozallowfullscreen="true" webkitallowfullscreen="true"></iframe>
   
</div>
<br>

Actually I just checked, and the embed code works perfectly in the index html file. But not in the markdown file unfortunately…

The browser I use is Firefox. In Edge the lines do not appear, but the blurrines of the text is still there…

Any idea what I can check to understand the problem?