Strange behavior using goavro.Union with map. (Question)

[fbalicchia]

I'm stumbling with some strange behavior using Union and I don't understand if I'm missing something in using API or if it is a possible issue. Please check the following piece of code

Thanks for support

package main

import (
	"fmt"

	"github.com/linkedin/goavro/v2"
)

const schema = `{
	"type": "record",
	"name": "appPublished",
	"namespace": "it.balyfix",
	"fields": [{
			"name": "appId",
			"type": "string"
		},
		{
			"name": "metaDataMap",
			"type": [
				"null",
				{
					"type": "map",
					"values" : ["string", "int", "boolean", "long", "double"]
				}
			],
			"default": null
		}
	]
}`

func main() {
	codec, err := goavro.NewCodec(schema)
	if err != nil {
		fmt.Println(err)
	}
	attemptOneKO(map[string]interface{}{"appId": "app1"}, codec)
	attemptTwoKO(map[string]interface{}{"appId": "app1"}, codec)
	attemptWithoutMetadataOK(map[string]interface{}{"appId": "app1"}, codec)
}

func attemptOneKO(message2Send map[string]interface{}, codec *goavro.Codec) {
	metaDataMap := make(map[string]string)
	metaDataMap["key1"] = "value1"
	metaDataMap["key2"] = "value2"
	message2Send["metaDataMap"] = metaDataMap
	buf, err := codec.TextualFromNative(nil, message2Send)
	if err != nil {
		fmt.Println(err)
	}
	fmt.Println(string(buf))

}

func attemptTwoKO(message2Send map[string]interface{}, codec *goavro.Codec) {

	message2Send["metaDataMap"] = goavro.Union("string", map[string]interface{}{
		"child1": "foo",
		"child2": "bar"})
	buf, err := codec.TextualFromNative(nil, message2Send)
	if err != nil {
		fmt.Println(err)
	}
	fmt.Println(string(buf))

}

func attemptWithoutMetadataOK(message2Send map[string]interface{}, codec *goavro.Codec) {
	buf, err := codec.TextualFromNative(nil, message2Send)
	if err != nil {
		fmt.Println(err)
	}
	fmt.Println(string(buf))

}

I'm using v2.12.0