They had named Johnson the winner by decision originally, and at the post fight press conference Dana White announced that, one of the judges fucked up his score card and the fight was actually a draw and Johnson didn't win it. And for this particular fight if there was indeed a draw finish to the fight then there would be a sudden death 4th round, but the mess up wasn't realized until it was too late.
There's going to be a rematch to this fight asap, as it's part of a tournament.