@NotTheNHK I see what you meant, but I think it's anti-intuitative to consider
that specifying @concurrent should have the same behaivor as if it's the implciit default.
I created it in Evolution forum because I thought this would require a language change, not only in implementation but also a consensus of the semantic of @concurrent wrapper of an actor-isolated closure, and the change might have other fallout. However, the discuss was more like an issue report than a proposal. So, yes, it should be in "Using Swift".
I don't think the issue should be left as is. But as an user a more pragmatic approach is a) not spending too much on it, b) knowing how to investigate such issues, and c) avoid pitfalls based on one's understanding.