XSLT 2.0 replace function behaves wrong

Question

The XSLT 2.0 template

<xsl:template match="/">
    <xsl:value-of select="replace('aba', 'a', 'b')" />
</xsl:template>

is expected to return 'bbb', which it does correctly when processed using saxonb-xslt on my Ubuntu 10.04 system. But when I issue exactly the same command on my Debian 5.0 system the output is 'abb'. In other words, the pattern is replaced only if it occurs at the end of the subject.

Both systems are standard setups (all software installed using apt, fully up-to-date), the version of libsaxon-java is 9.0.0.4 on both. On Ubuntu I am running sun-java6-jre, but I verified the output is also correct when switching to openjdk-6-jre (which is the JRE I am running on Debian).

I tried running saxonb-xslt with -ext:on on Debian but this had no effect.

Does anyone recognize this problem? I have run out of ideas how this might be possible.