> Maybe Rust is just not specified for every corner case?

Even in the absence of a formal specification, if you demonstrate undefined behavior in safe code, it will be regarded as a high-severity bug and slated for correction. If it's a bug in the compiler, it will be patched. If it's a bug in the language itself, the language will be redefined to prevent that behavior in safe code and the implementation will be updated to reflect this. If these changes break existing code, then so be it: soundness fixes are an instance where the Rust developers reserve the right to break backwards compatibility. Rust takes UB seriously.