This results in nested header elements for the site header, and because it's a pattern, updating the header-default.php markup doesn't update the markup for the existing site header.
I'd think <header class="wp-block-group"> should be changed to <div class="wp-block-group">
Updated to the latest wd_f, and the header template now eventually includes a pattern.
templates/index.html:
<!-- wp:template-part {"slug":"header","tagName":"header","className":"site-header"} /-->parts/header.html:<!-- wp:pattern {"slug":"wds/header-default"} /-->patterns/header-default.php contains<header class="wp-block-group" style="margin-top:var(--wp--preset--spacing--30);margin-bottom:var(--wp--preset--spacing--30)">This results in nested header elements for the site header, and because it's a pattern, updating the header-default.php markup doesn't update the markup for the existing site header.
I'd think
<header class="wp-block-group">should be changed to<div class="wp-block-group">