I found out that (as usual) MS decided to make IE10+ on touch devices behave differently from all the other mobile browsers. (sigh)
It does not support a hover on a li for a dropdown, even if there's no link but a span in it.
But.... you can add an extra attribute to the menu to "solve" this.

You can get support for IE10+ on touch devices, by adding the aria-haspopup attribute.

In XSLT of the DDR menu add this inside the li:

                    <xsl:attribute name="aria-haspopup">
                    <xsl:choose>
                        <xsl:when test="node">true</xsl:when>
                        <xsl:otherwise>false</xsl:otherwise>
                    </xsl:choose>
                    </xsl:attribute>