I wouldn't even mind wrangling some normalized data, but it doesn't seem very normalized in their examples.

Their first example suggests "great, there's a human appropriate title and detail, and maybe this standard will say you should at least have those and they should be ready for pass through to a human operator", with extensions providing room for more sophisticated behavior.

Then the second example, no more top level detail, now there's an 'errors' array, and detail is under the children (which they don't formally describe the concept of reparenting attributes, just incidentally showing it in an example of what an implementation could do with 'extensions'). Well, at least I can still pass through the details if I find them and it will make sense right? "must be a positive integer"... Ok, nope, error information that requires the client to process a json pointer in order to manufacture some sort of actionable feedback. Again, this could be a neat optional feature, but a generic core client really has nothing they can bite into that generically applies to the standard.

The cited RFC I think is close to some ideas but softens it by trying to be open ended. If it specified mandatory top level "detail" member that is reasonably directly informative to a human operator without further processing, great, I know exactly where to find it even if I don't otherwise understand your problem type. Mandate that errors may be a collection under an 'errors' list, but otherwise identical to top level? Cool. Saying that here's some recommended members, but they are all optional and the behavior is really up to you, and you can just freely change everything you want and call it 'extensions'... Just not prescriptive enough despite the long words...