What happens to the two First Order Storm trooper dice if they are still in the dice pool when Phasma is defeated?

Do they get removed like every other character or upgrade dice? Or do they remain in the pool and become the first dice in destiny history to be in a dice pool without being attached to an active card?

It’s just like Director Krennic and Seventh Sister’s Seeker Droid. Those dice are not tied to the original card once they are rolled in. If Phasma is killed, the FO Trooper die stays.

The card tells you what to do with the dice. They are set aside when removed or resolved. Phasma being defeated doesn't change that ongoing effect.