Boards are defined on a filter, which can select multiple projects. It is reasonable to expect that only epics from those projects would presented when looking at the list of epics available (currently it shows all epics from all projects). The suggestion is to have the list of epics grouped to show epics from the project the issue belongs to first, epics from projects which belong to a project within the board you are currently working on second and all other epics in a third group.