It's not rare that server maintenance will screw up peoples inventory. In this case, they saved the item you have but did not save the amount, or at least save it correctly. Giving 50 each of firecracker and gift box to every character is an attempt to say "sorry for the problem" but isn't fair to the players who had actually earned what they lost.
These event should be rule set to run between maintenances and after the maintenance, all items for the event will be gone. "Event over" and only the rewards would have to be saved correctly.
It's either that or they hire some smarter developers, ROFL.