Publish/Updating post is changing image links

I have written a filter to wrap images included in posts with figure tags and their captions with figcaption tags. I turned off the visual editor so that it doesn’t strip out all the html from the images.

That all seems to work fine until I publish or update the post. At this point the link to the full size image gets changed to /sitename/post-type/post-title/attachment/attachment-title/
where before it was /sitename/assets/filename.jpg

Can anyone help suggest why this is happening?

Thanks

Tim