All observed and experienced correctly, the cover aligns and keeps things from flying out. Some brands are better at it then others, but even the cutler hammer breakers will have parts pop out at times.
The part you have traced out in red was aligned, but, that you mentioned it its possible the common trip was not aligned as well as I though it was.
Hmm.. when I have time tomorrow. I'll open the 20A just like you did, or with smaller opening just to trip the bimetal.
Hope you are right, that the old common trip piece had flaw. Because if I'm right. Counterfeit GE breakers may be abound in the country (hundreds of thousands are sold here as it's the only Plug in and bolt on breakers we use).
I'd think both poles would be counterfeit if so. All the parts are assembled at a single factory.
Only way to know is to test.
Another possibility is a manufacturing defect. I've had GE breakers broken right out of the box here in the US.
Regarding counterfeits, only way to know is a certification from the manufacturer. Some counterfeits are obvious as day, others are indistinguishable from the real thing. Especially when a US company opens up a factory in China building parts to their spec. Then the contract runs out and everything is left behind- only for them to start making counterfeit good using the same molds, spec, logos, intellectual property, ect as a reputable manufacturer.