Under Section 22.2 fn:json-to-xml we write the following example:

<quote>
The expression json-to-xml('{"x": "\\", "y": "\u0025"}') returns one of the following: <map xmlns="http://www.w3.org/2013/XSL/json"> <string key="x">\</string> <string key="y">%</string> </map> or map{"x": "\","y": "%"}.
</quote>

It is about the last part:

> or map{"x": "\","y": "%"}.
But the signature of fn:json-to-xml only allows returning a document node. I don't think we allow it to return a map.