No, it is known why this would occur. It is a consequence of a new balancing mechanic that was added in Sun and Moon, where all moves that are affected by Prankster do not affect Dark-type Pokemon. Since Swagger gained priority from Prankster and was used on a part Dark-type Pokemon, the move didn't work due to this mechanic.
Whilst this does impede on Prankster's usefulness, it's still one of the best abilities in the game. You'll just have to be a bit more cautious in how you use it, since it's open to counter-play now (i.e. opponents can switch to a Dark-type Pokemon if they anticipate a Prankster move).