If the change is targeting a rust edition it's being treated as a feature/improvement not as a compiler bug. If there isn't a spec that explicitly says that this is expected behavior (and as far as I know there isn't a concrete spec for the rust language) it should be instead treated as a backwards compatible bug fix
No, it's being treated as a breaking change, which it is. It is not a backwards-compatible bug fix.
You are correct that there is no concrete spec for the Rust language; the current state of the compiler and stdlib is the "spec". So this is a breaking change to the "spec", and requires a new edition.