Not a defect. What is it with people equating "doesn't do this one hairbrained thing I want it to" with "broken?"
It's not a bug if it works as designed. Unless somewhere some official documentation says (some specific version of) grep supports what you're advocating for but the actual grep command doesn't, it's not a defect. It's a feature request.
To qualify as a "bug", I'd also accept "it used to do this and it doesn't any more and not on purpose".
Even if (say, GNU) grep maintainers decided they'd make grep support what you're going for, there'd still be design to do. Should it be a flag? Should the regex syntax be extended to support this? Should we add an environment variable? Some combination of the three? Something else? If we go with the flag, what should it be called and what should be its semantic meaning? Should it take an argument? Etc, etc, etc.
Even assuming this feature is necessary to fulfill "grep's intended purpose" (and I'm far from convinced it is), that doesn't make it a bug if it was never designed in to the program.