The battle also disappears if you leave afterwards to wait out repairs. Whatever their reasons it lands somewhere between petty and spiteful game design. Locking the retry for a day, having a five or ten core retry cost, or using the expedition damage system would have done.
I'm just going to hold out for an event where I can get some emblems. Leveling did nothing to help me progress in this event. I guess I'll need to level some defense tanks in other classes in case this ever shows up again. It's like this event will never end! Like a ghost event. Woo.