Ignoring Type Null, I think we must agree that if Gladion was going to get a 7th generation Pokemon, I think we have to agree it had to be Midnight Lycanroc. Its unfortunate the writers of the anime felt compelled to give Gladion a midnight Lycanroc because either Type:Null is a no-no (at the moment) or by sheer lack of variety that the games have been producing since 6th generation, 7th generation being slightly worse (being only made up by Alolan forms as well as legendary Pokemon).
So I feel like that's why Gladion is excused, as I said when Gladion was introduced, I hate that Gladion has a Lycanroc but did say it does make sense, as by design its the only thing that would suit Gladion (that's not named Type: Null)
Cross the movie exclusive character with Midnight Lycanroc or Lycanroc anything I call bullcrap on though. Sure Midnight Form does complement Incineroar if they were going with a theme, but honestly it only shows they are shoving 7th generation Pokemon just to some how justify retelling the story of how Ash got Pikachu, instead of a movie actually taking place in Alola, just as a 20th anniversary for the anime. In which case maybe it would've been better if Cross had a different 7th generation Pokemon than Midnight form.