Undefined properties in an example are undesired: if they aren't documented in the associated schema, it doesn't make sense to document them in the example. Undefined properties are usually just an error e.g. typo or after schema refactoring.

The validator could mark this as an error or warning, unless the schema has explicitly additionalProperties set, i.e. similar behavior as atlassian's swagger-request-validator ("validation.schema.additionalProperties" with default ERROR level).

Related discussion: belgif/rest-guide#146 (comment)

Note that the rule [req-valid] does allow undefined properties in responses only, but that's in a different context (API version compatibility vs API documentation)