It's a single post template which uses the Toolset image block to show the feature image for that post, and links to lightbox. I can give you access if you need it of course.
I did some tests on the site and found that the issue is stemming from the content template itself.
What I would like for you to do is to remake this content template completely. I've created a test template with the post below and as you can see the issue doesn't occur here. hidden link