We've been doing this since before 2.1, and saw the exact same thing as we see now. The behavior of the mobs is the same as pre-2.1. The behavior of players, however, tends to change as time goes on. The less time put into a fight, the less likely non-tanks are to jump the gun. You're probably only seeing it now because the rest of your group is more likely to start hitting abilities in that first 1-3GCDs.
[edit]
The only thing that may be a bug is if an add does literally go directly to a player that has done absolutely nothing at max range. I say may because it would be more likely to simply be an algorithm determining priority of targets when the aggro table is active, but no player is actually on it.