What version of Go are you using (go version)?

1.21.4

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

windows/amd64

What did you do?

Consider following code snippet:

func main() {
	type B struct {
		XMLName xml.Name
		C       bool     `xml:"c"`
	}

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

	a := A{
		B: B{
			C: true,
		},
	}

	output, err := xml.Marshal(&a)
	if err != nil {
		fmt.Println(err)
	}

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

Output in go 1.20.X

output: <a xmlns="http://example.com"><B><c>true</c></B></a>

Output in go 1.21+ (including 1.21.4)

output: <a xmlns="http://example.com"><B xmlns=""><c>true</c></B></a>

Remarks

This issue seems related to #61881 If one adds a tag to the XMLName field in B

	type B struct {
		XMLName xml.Name `xml:"b"`
		C       bool     `xml:"c"`
	}

then the empty namespace does not show up in the marshall result (in 1.21.1+).