The suddeness leaves unresolved feelings that will need to be addressed by them at some point in future. Meanwhile Delta is clearly focusing on her projects right now. That's a good thing for her and everyone! Exciting times!
IDK they both seem pretty happy right now. Nick seems to be very happy and back to his old self according to people who have met him, and Delta seems very excited to start this new phase of her career.
Strange, but maybe they were not as serious as we all thought. Regardless, I doubt they'll ever really see each other again at this point. I know they say they're staying friends, but all celebs say that when they break up. Heck Nick still calls Miley one of his best friends after their breakup but by all accounts they don't even speak anymore and haven't for years.