TestScenario1/2 are pretty much what we are discussing here - the container terminates and observers don't get notified any more.
Obviously, if you hang any additional 'thenRun' etc. on top of that, it won't work either.

TestScenario3 is with Weld parallel mode and is IMO out of scope of previous discussion but important nonetheless.

This is indeed weird and I think you are observing a peculiar behaviour of default executor in SE, which is ForkJoinPool.
We have a test[1] for parallel execution in SE, where we defined your own executor (see 'createWeld' method) and it works like a charm.
I think we need to look into FJP to see what's truly going on there.