Ha! I actually guessed correctly back when I theorized that the Masked King was really a part of Alucard.
Also, I always had a hunch that Miyabi was supposed to be special in some way. Like, we never even got so much of a hint as to what his true form was, did we?
But Miyabi actually being the Masked King and consequently Alucard himself indeed seemed a bit... sudden?
So I guess I can see how a few more hints throughout the story could've been preferable.
Although personally I'm not really bothered. I've seen better, I've seen worse. ¯\_(ツ)_/¯