You apply templates on the root node (or document node in XPath 2.0),
not the root element. So the template rule matching '/tasks' is applied, but its
caller never pass it a parameter. Its caller is not your apply-template, but
the default template rule.

Try instead:

   <xsl:apply-templates select="document('Tasks.xml')/tasks">
     <xsl:with-param name="project_id" select="record_id_"/>
   </xsl:apply-templates>