So, as of update 21 Quick Thinking causes constant lethal stagger locks from even weak sources of damage, including DoT effects.
At first glance this looks like a reversal of the fixes to Quick Thinking made in update 11 back in 2013. This when the Rage/QT combo was still really strong (for almost 2 years after), and when enemy levels in common missions where much, much lower.
The threshold for a stagger now appears to be as low as ~50 damage. Even a level 20 sapping osprey can stagger, and multiple overlapping sapping zones can mean certain death with no chance of escape.
In higher level missions surviving with low armour frames has become a matter of luck. A single unseen lowly trooper shooting through vegetation will stagger lock kill you. Vitals failing in JV results in stagger lock until death without a chance to acquire charges. The damage reduction from rolling and bullet jumping is no longer enough to prevent staggers.
This has honestly made the game almost unplayably frustrating with frames that aren't tanky or have strong defensive abilities. There are various frames that will get one shot by common enemies in the level 100 range, and now quick thinking mostly ensures that all energy will also be lost after having a very frustrating moment of complete helplessness.
If this change was intentional I don't understand the motivation. The game already suffers from a huge imbalance in EHP between different frames, and it's not like tanky/invisible etc. frames sacrifice something for the luxury, the list includes some of the all round strongest frames in the game. This just leaves squishy frames even further behind, while tanky frames still benefit from QT thanks to damage reduction preventing staggers.
This is definitely a step in the completely wrong direction. If this was a stealth nerf then I think we deserve an explanation and some clarity! If this is a bug then I think it should be fixed with the highest priority. Either way this is totally killing my enjoyment of the game. It's like having an ultra frustrating RNG death mechanic.
I'm aware that there is a thread where this topic is being discussed, however it did not specifically address the changes made in Update 21 and this causes a lot of confused arguing about separate issues.