@Gary Fawcett1  I was trying to think of a workaround solution as the event registry table is not available in scoped application. You found a workaround but this could still be used in some other solution.

You can still pass the event as string and use script in the "Fire Event" action to look for the event object.

// To be called in the Fire event action ehere event is selected.
var ev = new GlideRecord('sysevent_register');
ev.addQuery('event_name', fd_data.subflow_inputs.event);
ev.query();
if(ev.next()){
return ev;
}

Please mark the answer correct/helpful accordingly.