It has always occurred, you just haven't noticed.
Want to talk irritating? Start watching your front line melee characters. You will eventually see them latch on to a target and not let go no matter what you do - call a target 10 times, kite around, do anything but flag them (and that causes other troubles, nor could we flag the hench for most of the game). Then notice them kill said enemy, go stand by the target you called, and stand there. They stand there, again, no matter what you do. Call the target? - Nope, on their federally mandated smoke break. In fact, were you to watch even close they *all* do this, just it's usually hard to tell what the backliners are targeting. Is it a hero and you click a skill? Watch it flash for a good 10 seconds on a 1/4 second cast spell - WOOHOO!
I started around two months after release and they have *always* done this. I've learned not to fight it, for whatever reason they don't want to attack the target you are calling. I always just hope it happens *after* I engage the enemy - it also happens sometimes where they all stand around in a bunch while you run off, aggro the whole group, die, and then 5-10 seconds later they decide to engage and (maybe) rez you.
If it is just one of them then I ignore it. However I have had a few instances where *all* of them decide they don't want to kill said target and you are singularly wailing away on it by yourself. Best to switch and attack another, no amount of calling or anything will change the matter.
Worst case is that the sometimes all go off on their own and attack targets individually and there is *nothing* you can do about it except try and pick the target the largest majority of them have decided must die before they co-operate (or where the most AoE damage intersects).
My general feeling is that it is either intentional, if they actually followed your commands to the letter they would be *too* good (they are close to that now - we have to occasionally be reminded why PUGs suck so they mimic them). Either that or they don't consider the bug serious enough to fix, but then that goes back into the idea that fixing it makes them too good