Hi Jafula.

To add to your despair, I am experiencing similar problems as the people before me in this thread.
It is not only auto completion, but what appears to be a quite general problem with quest handling.
In my descriptions below, I sometimes use "click" as a reference to whatever function you use to select/accept/turn in/choose options etc.

I run wow on 1 or 2 computers (5 on 1 or 3+2), all with Jamba 6.2 and Keyclone as my program of choice.

No matter what setup I choose, the problem persists, and even though I've been trying hard to find some kind of logic in it, I'm really struggling to see any connections. The issues almost appear to be completely random, but it appears to get worse and worse the longer I play, and some times I even get error messages (turn-in only)

Quest pickup:
1: Select questgiver on all 5 characters and open interaction.
2: Choose desired quest (or only quest, makes no difference) on master
3: If the problem is there, some times, all slaves select the quest, some times, some slaves select the quest, but mostly none.
4: If the problem did not appear in step 3, some or all slaves will not "click" the accept button.
*** Some times, the slaves even auto collect the quests once I open the questgiver interaction. (But I have not tried your described solution for auto complete issues above)***

Quest turn-in:
1: Select questgiver on all 5 characters and open interaction.
2: Select quest to turn in
3: If there is a problem, some or all of the slaves may not select the quest
4: If reward, select reward and "click" confirm, else "click" confirm.
5: If there is a problem, some or all of the slaves may not select a reward, or select a reward but not "click" confirm.

As mentioned, I have seen error messages related to turn-in, and they all refer to some issues with quest handling, but I have been unable to force it to appear so I can post it here.. I will the next time it comes up.

In addition, and this is recently happened as well, but may possibly have some kind of relation.
I am using a "show-off" macro where my team speaks to eachother, using the /in x /s yyyyyyyyyy method, and towards the end of this macro, I am frequently getting error messages, and my timed /cheer at the end tends to get stuck on my screen. (Toon cheering continously)
To get it back to normal, I have to reload UI, which coincidentally is what I have to do to get the quest problem to go away for a short while too..

I hope I didn't bore you to death with my over-detailed description

Cheers
-Gizmo-