To me the expectation that Oxygen-28 would be stable was naive. If they had a model that said it would, I would have looked very askance at that model for one very simple reason: Oxygen-28 has never been observed in nature. If it hasn’t ever been observed before that’s a very strong clue it has a very short half-life.
Irony, thy name is … well, just check the user name. It’s all you need.