There is a misunderstanding on your side regarding the mirror function. Mirroring means that all configuration changes get mirrored from the master to all slave servers. This means that every newly added domain get added to all mirrors and all changes on these domains or subrecords get mirrored too. It does not mean that domains that were added before you installed the server get mirrored.
So there is no bug here. The problem with your setup is that you added a mirror server to a existing cluster that contained already records.
If you want to resync dns records on all nodes of a cluster, you can use the dns resync tool which is available in svn and can be used also with the ispconfig stable versions. Download the ispconfig svn version and then copy the files:
to the corresponding folders of your install on the master server. You will find a new menu item in the tools menu after you login again.