If the datamined stuff on the final phase of the fight is true, I would be beyond shocked if there isn't a cinematic.I havent done it but the general sentiment does seem to be that Mythic Xavius is undertuned and could have used a special mythic phase.
The lack of a cinematic isn't really odd, those typically have only happened for the final boss of the expansion. That said, it would be criminal if Guldan doesnt have one in Nighthold.