> Thus, even if this eventually deserves to be closed as a non-issue, I'd rather go on the safe side and double check if this is the desirable behaviour or if something went wrong with the commits previously mentioned. 

As Serhiy mentioned, this error message is raised from optimization path.
Nothing went wrong.  Error messages are vary depending on internal C code paths.  So this is not an bug or issue.  People must not rely on error message.

On the other hand, I don't oppose to making error messages more consistent.
It improve code health, like fixing typos in C comment which Python user doesn't see.