This is similar to the bug reported in; PM threads - Return to the beginning
In this case when someone posts the URL of a thread that will generate a Preview, that Preview is of the First Post in that thread and not the last post in that thread. In a long thread that only creates confusion and defeats the purpose of a preview.
This is a graphic of the preview.
Here is the preview chosen over the display of that URL. The above is an image captured of how it appears while the below may change if there is a change done to how the preview is generated.