Empty Thread after graft

Hi,

I'm using the forum module.
I created a new thread.
Added a few comments.
Pruned the Thread.
And grafted it in another Thread.

After that, the old thread should not be existend anymore.
But it is!
And when i click on it, i get a 404 (Page not found).

Could repeat it over and over.

Maybe i made a mistake.
But if this shouldn't work, there shouldn't be a link to prune the thread.

Sorry if this is a duplicate, but i didn't found a matching issue.