Codegen says that spec isn't valid that Validator calls valid

https://raw.githubusercontent.com/clemensv/spec-1/fix-schema2/message/schemas/xregistry_openapi_messagedefinition.json

I have this OpenAPI spec (which is part of the CNCF CloudEvents xRegistry project)

The Swagger Validator accepts is as valid, but the online version of Swagger Generator calls it invalid, but with no detail. I am assuming the validator is correct as this spec also works with other tools.

{  "code": 1,  "type": "error",  "message": "The swagger specification supplied was not valid"
}