Why should this be a surprise? The reason why Deadpool was R-Rated was because the creative staff targeted that rating from the very get-go. A movie that wasn't designed to do so from the beginning has very little reason to go for that rating halfway through production. An R-Rated Suicide Squad might well be better than a PG-13 one, but they would have had to have that goal at the very start of production for it to make any difference.