Usage of Asterisk in XPath expressions

due to a typo, we missed the colon between the namespace prefix and the
wildcard in a XPath expression. To my surpise, this causes static errors
in some version of Saxon and runs flawlessly in others, although I would
expect this to be static error.

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
B B B  xmlns:foo="http://test/ns/foo"
B B B  version="2.0">

B B B  <xsl:template match="foo:bar">
B B B B B B B  <xsl:if test="ancestor-or-self::foo*[@a]">
B B B B B B B B B B B  <xsl:copy>
B B B B B B B B B B B B B B B  <xsl:value-of select="."/>
B B B B B B B B B B B  </xsl:copy>
B B B B B B B  </xsl:if>
B B B  </xsl:template>

As you can see the colon is missing after the namespace prefix foo. What
baffles me is that different versions of Saxon exhibit different
behavious here, and especially newer versions don't seem to treat this
as a problem at all.

 * Saxon 9.6: Stops with something that looks like an internal error
 * Saxon 9.7: Says I need to enable XPath 3.1 to use this expression?!?
 * Saxon 9.8, 9.9: Don't run into an error at all

That makes me wonder: Is this an error at all? or is there some wildcard
feature or something in XPath 3.1 that I cam't find in the docs?