I tinkered with it a bit more this morning, and managed to nix the post mortem issue. (I was correct in that the issue was connected with the doubling of events, i.e. Fire and Earth weaknesses being triggered on same event)
Not sure how it was fixed, but the events are seeming to be triggered more than once... which is good.
However, I can across a new issue: Water Spirit cast Water X on Actor 1; Fire Spirit's Weakness was triggered, Fire Spirit has been stunned.
Somehow the events are acting as universals conditions, so that even if mob/actor is not the target of an opposing element, the event is still being triggered because a spell/skill is being used which is linked with that event.