This is fixed - artifact left over from the beta testing. Not sure about your other issue though regarding only new posts?? That doesn't seem correct - you sure about that?
I think you were still working on it or something, but now it works sorta correctly on one of my own threads:
Only issue was when I opened the post I wanted to create directly, since I had them already linked in the second post the Shortcut button did not appear. I had to search for them in the thread.