IT wasn't that Astrid wanted to hide his connection to Wulfric. He simply needed more time. Time to build his career properly, time to make sure his fame stretched across all four galaxies. Once he'd achieved that, once his name carried enough weight on its own, then letting the world know about them wouldn't matter. At least then, fewer people would dare claim he wasn't worthy of standing at Wulfric's side.
Some might argue he shouldn't care what people think. But this wasn't just about opinions. Astrid refused to become someone who gave others a reason to criticize Wulfric. Because the fact still remained that in everyone's eyes, their positions in society were still worlds apart.
Of course, revealing he was Duke Grimaldi's grandson would instantly change his standing in society. But he didn't want that to happen, not until the Lancasters were brought down. There were just too many risks in exposing his connection to the Grimaldis right now.