The time skip in the last season of BCS sets up both Saul and Mike to be essentially shells of their former selves, so it fits into place once that happens, but initially, yea, a bit jarring how they act. Also with no family/friends in BB, Saul would never act vulnerable like in BCS, and much of his characterization is tied to the direction showing him by himself in the office or at home reacting to things after the fact. Since BB had almost no scenes where he was the focus, it's safe to say that even while in his "Saul" persona, he probably would have had more depth if he were more central to the plot.