I wonder if the reason it works is because you'd easily come across it in normal code, and someone didn't realize that saving the effort of implementing a check for this would result in unsoundness.

That's not how it works. rustc doesn't "check" for specific cases explicitly, the borrow checker (apparently at fault here by reports) has a set of rules that should reject invalid programs. So either one of the fundamental rules was broken (unlikely here from what I've heard), or some optimization caused the rules to not be implemented correctly.