grouping confusion

As I've reworked my stylesheets, I've gotten stuck on some code that
was previously working.  It formats a citation by doing a multi-level
grouping and sorting by author and then year.  If there is more than
one author, only the first should be printed.  Likewise, if there's
more than one author-year combination, then only the suffix gets
printed.  Example:

It's clear the problem is in how I'm using the position() function
within the groups, but I can't figure out how to fix it.  I suspect it
may be something about the bibref variable.  In the previous approach,
I had used a key for that indexed all the bibliographic records by
their ID attribute, while here I'm just accessing a variable that holds
all those records.  Is there something different conceptually about the
two approaches that would cause position() to behave differently in
this context?

  <xsl:template match="db:citation">
    <!-- store citation for future use -->
    <xsl:variable name="citation" select="."/>
    <xsl:variable name="idref" select="db:biblioref/@linkend"/>
    <xsl:variable name="bibref"
select="$enhanced-biblist/mods:modsCollection/mods:mods[@ID=$idref]"/>
    <xsl:value-of select="$style-citation/cs:prefix"/>
    <xsl:choose>
      <xsl:when test="$citeclass='author-year'">
        <xsl:for-each-group select="$bibref"
group-by="bib:grouping-key(.)">
          <xsl:sort select="current-grouping-key()"/>
          <xsl:for-each-group select="current-group()"
group-by="bib:year">
            <xsl:sort select="current-grouping-key()"/>
            <xsl:variable name="position" select="position()"/>
            <xsl:for-each select="current-group()">
              <a href="#{@ID}">
                <xsl:if test="position() = 1">
                  <xsl:choose>
                    <xsl:when test="mods:name">
                      <xsl:apply-templates select="mods:name"
mode="short"/>
                    </xsl:when>
                    <xsl:otherwise>
                      <xsl:value-of select="bib:noname-substitute"/>
                    </xsl:otherwise>
                  </xsl:choose>
                  <xsl:value-of
select="$style-citation/cs:entry/cs:creator/cs:suffix"/>
                  <xsl:value-of select="bib:year"/>
                </xsl:if>
                <xsl:apply-templates select="bib:key"/>
    <!--+ ==============================================================
    | render point citation details if present
    +-->
                <xsl:if test="$citation/db:biblioref/@begin">
                  <xsl:value-of
select="$style-citation/cs:entry/cs:point/cs:prefix"/>
                  <xsl:value-of select="$citation/db:biblioref/@begin"/>
                  <xsl:if test="$citation/db:biblioref/@end">
                    <xsl:text></xsl:text>
                    <xsl:value-of
select="bib:number-condense($citation/db:biblioref/@begin,
$citation/db:biblioref/@end)"/>
                  </xsl:if>
                </xsl:if>
              </a>
            </xsl:for-each>
            <xsl:if test="$position != last()">, </xsl:if>
          </xsl:for-each-group>
          <xsl:if test="position() != last()">
            <xsl:value-of select="$style-citation/cs:entry/@delimiter"/>
          </xsl:if>
        </xsl:for-each-group>
      </xsl:when>
      <xsl:otherwise>
        <xsl:value-of select="$style-citation/cs:prefix"/>
        <xsl:for-each select="db:biblioref">
          <xsl:variable name="linkend" select="@linkend"/>
          <xsl:variable name="number">
            <xsl:choose>
              <xsl:when test="$sort_order-bib='cited'">
                <xsl:value-of
select="$cite-position/bib:refs/bib:unique/bib:ref[@key=$linkend]/
@position"/>
              </xsl:when>
              <xsl:when test="$sort_order-bib='citekey'">
                <xsl:value-of select="$linkend"/>
              </xsl:when>
              <xsl:otherwise>
                <xsl:value-of
select="$formatted-biblist/xhtml:p[@id=$linkend]/xhtml:
span[@class='mark']"/>
              </xsl:otherwise>
            </xsl:choose>
          </xsl:variable>
          <a href="#{$linkend}">
            <xsl:value-of select="$number"/>
          </a>
          <xsl:if test="position() != last()">, </xsl:if>
          <xsl:if test="position() != last()">
            <xsl:value-of select="$style-citation/cs:entry/@delimiter"/>
          </xsl:if>
        </xsl:for-each>
        <xsl:value-of select="$style-citation/cs:suffix"/>
      </xsl:otherwise>
    </xsl:choose>
    <xsl:value-of select="$style-citation/cs:suffix"/>
  </xsl:template>