> Is it that Specta/Expecta can't handle this assertion on another thread while it's running the run loop to handle the `wait` expectation?

Not sure. If you'd be willing to dig into that, maybe we could submit a patch upstream (but no worries if not).

> What do you mean by "different asserting tool"?

Assumably SenTestingKit wouldn't have the same problem, although I don't know that with certainty.