Ok, so I've just checked what's happening on precise, and it seems that new LO desktop files include the StartupWMClass key.
This is ok and Bamf can use it for matching, the problem is that the LO windows are slow on updating their window class and this leads to a matching error in precise (this won't happen in saucy instead).
A woarkaround for people affected by this is just removing that key from files by running:
sudo sed '/StartupWMClass=.*/d' -i /usr/share/applications/libreoffice-*.desktop
A code fix will be released as well. Move to bug #1238122 to track this.