If it's something akin to Sugars Devil Fruit, then EVERYONE forgets about it. A devil fruit ability isn't going to be selective like "Oh, that's a WG official, he's cool, he can remember."
Who knows, it could all be apart of the Gorosei's plan. They spoke about a Great Cleansing. Perhaps erasing Lulucia wasn't to make an an example, but rather root out all the other Countries who could potentially conspire against them. That would actually be pretty smart of them, tbh. But at the same time, you can't really predict what would happen. It could also be a huge miscalculation, that just ended up backfiring.