It's not a bug. You need to learn the art of debuging and such.
I have used test map from topic you have had linked earlier.
The fact that usage of GetWidgetLife condition removed the problem shouldnt have suprised you nor anyone else who looked at your group creation criterie. Yeah, there is none.
You kill dummies instead of removing them. While multiple Chain Fires are under way, multiple dummies appear, thus they are added to target groups and since those are killed and the end of effect their life becomes 0 and are still aviable targets for a couple of seconds. Since FirstOfGroup isn't precise function, it will mostlikely target dummies and with Shift method the chance of actually targetting dummies rises (more dummies on map).
There are several ways to eliminate the problem - and GWL it's the most accurate one. You need to add conditions for unit to be added into group. I have entered additional debug messages to let you know where the problem lies. If you still will be unable to fix your problem with this knowledge, let us know in this topic.
By the way - bump is better option then creating new thread.