?variant_id = in marketing feeds (variant:id is not the variant_id, it is the variant product's ID)

variant_id.xml

<mvt:if expr="NOT l.settings:record:variant:variant_id">
    <mvt:do name="l.settings:record:variant:product_part_count" file="g.Module_Library_DB" value="ProductVariantPartList_Load_Part(l.settings:record:variant:id, l.settings:record:variant:productvariantparts)" />
    <mvt:if expr="miva_array_elements( l.settings:record:variant:productvariantparts ) GT 0">
        <mvt:foreach iterator="pvp" array="record:variant:productvariantparts">
            <mvt:if expr="l.settings:pvp:product_id EQ l.settings:record:id">
                <mvt:assign name="l.settings:record:variant:variant_id" value="l.settings:pvp:variant_id" />
                <mvt:foreachstop />
            </mvt:if>
        </mvt:foreach>
    </mvt:if>
</mvt:if>
<mvt:assign name="l.settings:record:variant:canonical_uri" value="l.settings:record:canonical_uri $ '?variant_id=' $ l.settings:record:variant:variant_id" />