I don't think it's always been this way. But recently I've noticed it hitting targets outside of the staircase(which is already an absurdly long distance already). It's less so about diving close to the fountain, but about dragging waves of minions to it. A smart team can get wave after wave of minions to follow them into the fountain without the nexus turret to even target any.
There is also a definite bug where the turret will attack you if you hug the wall by the fountain. It's hard to explain, but it shouldn't be happening.
This bug makes it very difficult to win a game if the enemy turtles all game. I haven't experienced a turn around from it, but I have had the scenario where what should have been a 20-30 minute game ended at 50 minutes.