Saxon

While experimenting with streaming I have run into an odd message (using Saxon EE 11.4 J):

Error in xsl:for-each/@select on line 15 column 40 of value-of-test3.xsl: SXST0060 Component cannot be streamed, although it is guaranteed streamable according to W3C rules. No streamable path found in expression (xsl:value-of, ...)

The code is

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="3.0"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  exclude-result-prefixes="#all"
  expand-text="yes">

  <xsl:strip-space elements="*"/>

  <xsl:output method="text"/>

  <xsl:mode streamable="yes"/>

  <xsl:template match="/" name="xsl:initial-template">
    <xsl:for-each select="root/record">
      <xsl:value-of select="data(@att1), data(@att2), if (normalize-space(@att3)) then @att3 else 'default'" separator=","/>
      <xsl:text>&#10;</xsl:text>
    </xsl:for-each>
  </xsl:template>
  
</xsl:stylesheet>

XML sample

<root>
  <record att1="a1" att2="b1" att3=""/>
  <record att1="a2" att2="b2" att3="c2"/>
  <record att1="a3" att2="b3" att3="c3"/>
</root>

Is that a Saxon quirk during streamability analysis and/or streamed execution or is that construct really too complicated?

I started with something like <xsl:value-of select="@att1, @att2, if (normalize-space(@att3)) then @att3 else 'default'" separator=","/> and even there I thought with only attribute nodes being used that value-of would be streamable but Saxon rejected it.

Then I tried to simplify the task for Saxon, throwing in data calls but got the above strange message.