This article explains what happens when you move an issue that is synced with Backbone to a different project.
When you move the issue to another project, the following will happen:
Backbone will not throw an error (i.e issue is missing)
Backbone will synchronize changes made on the issue which was not moved to the issue which was moved
You can unlink the issues if you don't want the changes to be synced anymore
Backbone will not synchronize changes made on the issue which was moved
If you are moving an issue to a project with a synchronization: Backbone will also sync the moved issue to a new issue and keep those two in sync according to the new synchronization. Please note that backbone sync panel info might be outdated after moving an issue if the steps (5.5) Move a synced issue to a new project are not followed.