Nonsense
The turret as well as other devices
except the Steel Mosquitos, attack the closest mob
OR the one you have the aggro from and/or are engaged in combat with.
after reading the posts on the forums I tried to reproduce the same results some players were having and was unable to do so. As far as I can tell, the devices operate as intended and any issues that appear are minor glitches but possibly and mostly human error.
I believe that what happens though is players will place the turret where it's presence will draw the aggro before it even has a chance to fire a shot off. This can make it seem the turret isn't working as intended because it will immediately attack the mob attacking it before it switches to your current mob. As far as pulling multiple mobs, well, we all do that. If they're close you'll most likely get the entire pack of mobs.
I'm sure they'll be making future enhancements to not only the engineer but other classes as well. Just give it some time. Things may change, if not, at least you'll be more experienced and have a better hang of things on the working of the class.
Good luck!