[BUG] Gradle plugin seems to mistake JSON OpenAPI 3.0 spec for Swagger 2.0, while CLI works fine

Running the Gradle plugin on the JSON file obtained from the online OpenAPI 3.0.1 definition exposed by Springdoc fails validation, but generates the client stubs fine with the openapi-generator-cli-5.1.1.jar.

My configuration is simply like this (I tried with other languages not just dart with the same effect)

buildscript {
    repositories {
        mavenLocal()
        mavenCentral()
    }
}

plugins {
    id "org.openapi.generator" version "5.1.1"
}

openApiGenerate {
    generatorName = "dart"
    inputSpec = "$rootDir/api_spec/api.json"
    outputDir = "$rootDir/../api"
    apiPackage = "mypackage.api"
    modelPackage = "mypackage.api"
}

However I get a bunch of errors like this:

> Task :openApiGenerate FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':openApiGenerate'.
> There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
   | Error count: 72, Warning count: 0
  Errors:
        -attribute paths.'/subscriber/{id}/device'(post).[id].type is missing
        -attribute openapi is unexpected
        -attribute servers is unexpected
        -attribute swagger is missing
        -attribute components is unexpected

Why is it expecting swagger when this is an OpenAPI 3.0 definition, and complaining that openapi is unexpected?
This file works perfectly with CLI generator but for some reason Gradle is not detecting it as OpenAPI 3.0.1.

This seems to be a bug with the plugin.

Reproduce the failure with api_spec/api.json and the Gradle openApiGenerate configuration using plugin version 5.1.1, then compare it with openapi-generator-cli-5.1.1.jar. Trace how the Gradle plugin selects validation and specification handling; done means the OpenAPI 3.0.1 document validates and generates the client successfully through Gradle.