Go version

go version go1.21.6 darwin/amd64

Output of go env in your module/workspace:

N/A

What did you do?

package main

import (
	"encoding/xml"
	"fmt"
)

func main() {
	type B struct {
		XMLName xml.Name
		ID      string `xml:"id"`
	}

	type A struct {
		XMLName xml.Name `xml:"http://example.com a"`
		B       B        `xml:"b"`
	}

	output, err := xml.Marshal(&A{B: B{ID: "1"}})
	if err != nil {
		fmt.Println(err)
	}

	fmt.Printf("output: %s", string(output))
}

Playground example here

What did you see happen?

Output in go 1.21.6

output: <a xmlns="http://example.com"><b xmlns=""><id>1</id></b></a>

What did you expect to see?

Output in go 1.20

output: <a xmlns="http://example.com"><b><id>1</id></b></a>

It's similar to #61881, but this case it's like a bad use of XMLName tag, this break for us but can be fixed removing the XMLName tag not used, but previous versions simply ignored that