hook_xmlsitemap_link_alter never seems to get invoked

I'm trying to modify the urls produced by the taxonomy module to point to facets instead of term pages (should be trivial as I just need to add /search to the start of every url), but I can't seem to get hook_xmlsitemap_link_alter to ever be invoked.

I drop this in my custom module (ots_custom.module) just to test if it gets invoked, but rebuilding, enabling, disabling; I never see the dsm and they never get changed to priority 1.

function ots_custom_xmlsitemap_link_alter(array &$link, array $context) {
dsm($link);
if ($link['type'] == 'taxonomy_term') {
$link['priority'] = 1;
}
}

What am I missing? Thanks!