--fix-suggestions rewrites this one, and a plain --fix leaves it alone: the correction changes what the stylesheet does, or is one of several the check would accept.

Leaking result namespace

A literal result element copies every namespace in scope into the result, so a prefix declared only for the stylesheet's logic (xs for as="xs:integer", a prefix for your own functions) is serialized onto output elements that never use it. List the prefix in exclude-result-prefixes on the stylesheet root: expressions still use it, and the output no longer carries it.

Incorrect:

<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xsl:template match="/">
    <object>
      <xsl:value-of select="1 cast as xs:integer"/>
    </object>
  </xsl:template>
</xsl:stylesheet>
<!-- <object> is serialized as <object xmlns:xs="..."> -->

Correct:

<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  exclude-result-prefixes="xs">
  <xsl:template match="/">
    <object>
      <xsl:value-of select="1 cast as xs:integer"/>
    </object>
  </xsl:template>
</xsl:stylesheet>

A simplified stylesheet, whose root is itself a literal result element, spells the attribute xsl:exclude-result-prefixes.