Template "sitemap_index_xml":
<loc><t t-esc="url_root"/>sitemap-<t t-esc="page"/>.xml</loc>
should be:
<loc><t t-esc="url_root"/>sitemap-<t t-esc="website_id"/>-<t t-esc="page"/>.xml</loc>
The fix in python is not elegant but allow to fix without -u of website.