Always pulling first item

No matter what I do, random_content always appears to display the same item.

Carefully deleting and adding items and then reloading the page suggests that it’s always displaying the first (earliest created) content item that matches the specified group ID.

It doesn’t appear to be a cache issue, category grouping seems to be working properly, I’ve tried different browsers and different computers and different IP routing to my server.

Any clue how I should investigate? (I’d be delighted to find out it’s user error and I’m doing something dumb.)