One of my customers uses k15t's Backbone Issue Sync extensively - Jira (& Servicemanagement) Data Center. In at least one Sync we get the following error message:
The epic is not under sync or has not been synced yet.
I checked: The Sync is simply set to copy the Epic Link field from Issue A (source) to Issue B (target). The rest of the Sync evidently works normal.
Normally, Issues don't need to be in the same Project as their Epic. I couldn't find anything in the official documentation nor in the Community.