When flying against invader, Opes will successfully kill the first four groups of five, but then, Southern Cross stays “aglow” but you cannot select it, and it cannot be fired.
I have not tried other dragons right now. It is not a spell combo, since I do not use any other spells when running Invader.
It is possible that it is related to firing Southern Cross, and then swiping for a normal attack after SC has hit. If I tap (tay to hit a monument or something that will die with one hit), the stuck SC doesn’t seem to happen, but firing, and then swiping, and then trying to use SC again does not work properly. It could be timing, but I have also tried spacing it out more, and it did not make a difference.