I know that in SwSh, certain raid Pokemon could have more than 4 moves as a way to make them more difficult, including events like Mewtwo as well as most legendaries in Dynamax Adventures. With SV, it feels like they were trying to increase the difficulty ceiling for raids, so I wouldn't be surprised if 6 and 7-star bosses have more than 4 moves.
If caught, the Pokemon go back to only knowing 4 moves.