I tried this triumph by keeping it at 100% and finished it as heroic event with the clan members but still couldn't get the triumph. I've noticed that at the beginning of the public event, rift generator core's health is already under 70% when spawned. But the triumph's description says 'Complete the "Rift Generator" public event without allowing the Generator Core to drop below 100% integrity'. Our clan members came to conclusion that this triumph is still bugged and decided not to try this anymore. I know that developers were aware of this issue but It would be nice if bungie can confirm if this is still bugged or not.