There is nothing wrong, it is working as intended.
The new graphics only appear when the link has , in other cases it remains normal.

This is because what gives the order to the browser to apply another graphic is the address ending with #idpost.
Because ;topicseen#new varies from post to post. In other words, it always points to the last post, regardless of the author of the post.

It's true that this could be adjusted, but I don't think it's that relevant.