We found that if a topic's posts contained images pointing to a relative path (e.g. ![test](1.jpg), NodeBB would attempt to load them, causing them to load the entire topic again (since the url is site.com/topic/123/1.jpg).
To combat this, @baris restricted the URL so that the slug must be correct, otherwise the user is sent to 404.
For lack of a better solution, this was the approach we took, so this seems to be the reason why you're running into this issue now.