Everything working but image url

[mac2net]

Hi
(I also posted this on wp.org/supprt)
Everything is working apart from modifying the image url in the post.
The post is uploading, the images are uploading and the frontmatter is working.

But the url of the images doesn’t change from…
_images/
…to the location of the image.

I have the Classic Editor set as default. Here is the code in the post for an image…

<figure class="wp-block-image size-full">
<img src="_images/image-file-name.png" alt="" />
</figure>

I checked here and on GitHub and I couldn’t find a solution to the problem. Could it perhaps be a conflict with another plugin.

Cheers
Mac2Net