That's probably because you aren't using X.org's clipboard manager but something different like Klipper (KDE) or Gnome clipboard manager. It's kinda easy to imagine how annoying it can be: You copy an url from a browser and close it and then try to paste it but woops... you can't. You try to go back but you can't remember what the url was. Have fun finding it again.
It's called "browser history" ... in any case, yeah, this is an annoying bug