When a code block contains syntax for an image, lemmy seems to replace the image's url with a proxied one.
This instance's web UI's preview displays the original url correctly. The proxied url is also distributed trough federation, so users on other instances or for example Piefed should also see the lemmy.ml url in the example below.
This behavior is unexpected, as the contents of a code block should not be parsed as markdown, and thus no image should be displayed and no requests to the server should be made.
Example:
I entered "https://media.piefed.zip/posts/bT/UD/bTUDGZRAcKZdXyO.png" as the url in the following code block in the web UI: 
Lemmy UI version: 0.19.17
Lemmy backend version: 0.19.18-beta.0