The problem arose not because the backstab couldn't be seen coming. It's that the reasons you could see it coming conflicted with my basic understanding of Quinn's personality. And since, you know, our companions' personalities are hammered into us with every bit of dialogue we choose, that sort of thing is fairly important. Quinn was a spy, but the reason he was a spy sucked. That's what was most annoying about the whole thing.
A spy isn't going to reveal their true personality to you when talking to them. The entire point of a spy is to present to you a fiction that you'll believe that makes you want to trust them.
Of course Baras is an accomplished liar. That doesn't matter at all in this particular instance. In fact, it could, and ought to, work against him. Quinn's debt to Baras was based on a sense of honorable debt. It would be bizarre for him to agree that this debt remains in operation for the sake of a lie. So, what, Baras calls in after Voss and says, "oh, by the way, I lied, you still totally owe me"?
Baras telling Quinn "You are free from my debt" isn't a message to Quinn, it's a message to you. I'm not sure you understand how spies and lying to somebody works to get an agent into their midst. The Agent knows the lie that is going to be run the entire time. Baras is selling the ground work for Quinn working for you, not to Quinn, but to you. Quinn knows all the time who his true master is.
But then again, the entire Quinn line is based on that sort of unfathomably strange connection between the man's "honor" and his service to an ideal that is completely execrable and contemptible. No normal understanding of "honor" could justify Quinn's continued service to Baras, but Quinn does not possess a coherent and normal understanding of "honor". That's what makes him a poor character. You don't really know where he's going to jump next.
He gave his loyalty to Baras first. It doesn't matter how much stuff you do "for him" after that, he's loyal to his original master and is going to do what he told him he was going to do.
The entire "But I did more stuff for him more recently so he should be loyal to me more!" Shows that you completely don't understand how true loyalty works. You don't bribe off loyalty by doing more good stuff for somebody then the person they are loyal to does. If they switch sides because of that they weren't loyal to their original master.
Add on: As a side I don't think that Quinn has that big of a problem with people lying to complete a mission. I know I've had my character tell some huge lies to people to get what he wants and most of the time Quinn just stands there without caring. No -1 or anything.