I have a file that I ran through the function/example I sent to you on my PR:

<?xml version="1.0" encoding="utf-8"?>
<!-- Generator: Adobe Illustrator 23.0.4, SVG Export Plug-In . SVG Version: 6.00 Build 0)  -->
<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" width="24px"
	 height="24px" viewBox="0 0 24 24" enable-background="new 0 0 24 24" xml:space="preserve">
<g id="Bounding_Box">
	<rect fill="none" width="24" height="24"/>
</g>
<g id="Icons">
	<g>
		<g>
			<path d="M2.5,19h19v2h-19V19z M19.34,15.85c0.8,0.21,1.62-0.26,1.84-1.06c0.21-0.8-0.26-1.62-1.06-1.84l-5.31-1.42l-2.76-9.02
				L10.12,2v8.28L5.15,8.95L4.22,6.63L2.77,6.24v5.17L19.34,15.85z"/>
		</g>
	</g>
</g>
</svg>

When it called Encode on it this was the output:

<?xml version="1.0" encoding="UTF-8"?><><><!--Generator: Adobe Illustrator 23.0.4, SVG Export Plug-In . SVG Version: 6.00 Build 0)--></><svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" width="24px" height="24px" viewBox="0 0 24 24" enable-background="new 0 0 24 24" xml:space="preserve"><g id="Bounding_Box"><rect fill="none" width="24" height="24"></rect></g><g id="Icons"><g><g><path d="M2.5,19h19v2h-19V19z M19.34,15.85c0.8,0.21,1.62-0.26,1.84-1.06c0.21-0.8-0.26-1.62-1.06-1.84l-5.31-1.42l-2.76-9.02&#xA;&#x9;&#x9;&#x9;&#x9;L10.12,2v8.28L5.15,8.95L4.22,6.63L2.77,6.24v5.17L19.34,15.85z" fill="#f44336"></path></g></g></g></svg></>

Notice the <> and the </> are invalid XML. I even just did a Parse and then an Encode and got the same results, so its not my function for WalkMutate at all its a bug elsewhere:

				xt, err := fxml.Parse(f)
				if err != nil {
					return
				}

				var buf2 bytes.Buffer
				err = xt.Encode(&buf2, true)
				if err != nil { 
					return
				}
				xmlString2 := buf2.String()
				session_functions.IsolatedDump(xmlString2)

So as a hack I am just replacing these with nothing and it fixed this one icon until you can get back with me on why you think it's parsing it incorrectly after it encodes it:

				var buf bytes.Buffer
				err = newXML.Encode(&buf, true)
				if err != nil { 
					return
				}
				xmlString := buf.String()
				xmlString = strings.Replace(strings.Replace(xmlString, "</>", "", -1), "<>", "", -1)
				data = []byte(xmlString)

Callers are expected to return x as well as the byte for whether to continue or stop

Example below can be added to docs later if you want:

	var filename = "forward_10-24px.svg"
	f, err := os.Open(filename)
	if err != nil {
		return
	}
	defer f.Close()
	xt, err := fxml.Parse(f)
	if err != nil {
		return
	}
	newXML := xt.WalkMutate(func(ni fxml.XNodInfo, x *fxml.XMLTree) (fxml.XWalkResult, *fxml.XMLTree) {
		if ni.Path[len(ni.Path)-1] == "path" {
			addFill := true
			for idx, _ := range x.Attr {
				if x.Attr[idx].Name.Local == "fill" && x.Attr[idx].Value == "none" {
					addFill = false
				} else if x.Attr[idx].Name.Local == "fill" {
					x.Attr[idx].Value = "red"
					addFill = false
				}
			}
			if addFill {
				x.Attr = append(x.Attr, xml.Attr{
					Name: xml.Name{
						Local: "fill",
					},
					Value: "red",
				})
			}
		}
		if ni.Path[len(ni.Path)-1] == "circle" {
			addFill := true
			for idx, _ := range x.Attr {
				if x.Attr[idx].Name.Local == "fill" && x.Attr[idx].Value == "none" {
					addFill = false
				} else if x.Attr[idx].Name.Local == "fill" {
					x.Attr[idx].Value = "red"
					addFill = false
				}
			}
			if addFill {
				x.Attr = append(x.Attr, xml.Attr{
					Name: xml.Name{
						Local: "fill",
					},
					Value: "red",
				})
			}
		}

		if ni.Path[len(ni.Path)-1] == "polygon" {
			x.Attr = append(x.Attr, xml.Attr{
				Name: xml.Name{
					Local: "style",
				},
				Value: "fill: red;",
			})
		}

		if ni.Path[len(ni.Path)-1] == "rect" {
			addFill := true
			for idx, _ := range x.Attr {
				if x.Attr[idx].Name.Local == "fill" && x.Attr[idx].Value == "none" {
					addFill = false
				} else if x.Attr[idx].Name.Local == "fill" {
					x.Attr[idx].Value = "red"
					addFill = false
				}
			}
			if addFill {
				x.Attr = append(x.Attr, xml.Attr{
					Name: xml.Name{
						Local: "style",
					},
					Value: "fill: red;",
				})
			}
		}
		return fxml.WRCont, x
	})
	newXML.Encode(os.Stdout, true)

Here is the forward svg file:

<?xml version="1.0" encoding="utf-8"?>
<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" width="24px"
	 height="24px" viewBox="0 0 24 24" enable-background="new 0 0 24 24" xml:space="preserve">
<g id="Bounding_Box">
	<rect fill="none" width="24" height="24"/>
</g>
<g id="Icons">
	<g>
		<path d="M18,13c0,3.31-2.69,6-6,6s-6-2.69-6-6s2.69-6,6-6v4l5-5l-5-5v4c-4.42,0-8,3.58-8,8c0,4.42,3.58,8,8,8s8-3.58,8-8H18z"/>
		<polygon points="10.86,15.94 10.86,11.67 10.77,11.67 9,12.3 9,12.99 10.01,12.68 10.01,15.94 		"/>
		<path d="M12.25,13.44v0.74c0,1.9,1.31,1.82,1.44,1.82c0.14,0,1.44,0.09,1.44-1.82v-0.74c0-1.9-1.31-1.82-1.44-1.82
			C13.55,11.62,12.25,11.53,12.25,13.44z M14.29,13.32v0.97c0,0.77-0.21,1.03-0.59,1.03c-0.38,0-0.6-0.26-0.6-1.03v-0.97
			c0-0.75,0.22-1.01,0.59-1.01C14.07,12.3,14.29,12.57,14.29,13.32z"/>
	</g>
</g>
</svg>

Also added a bug fix for the lower cased utf-8 example.

Nice job on this. Super useful package for parsing SVGs which I needed to do here.