And actually, I already had a solution for this in the Relevanssi Knowledge base.
“There’s something of a compatibility issue between Relevanssi and Gutenberg Full Site Editing. No results are found when you create a Query Loop to display the search results with Relevanssi enabled. The blank results happen because the post template block doesn’t use the Relevanssi results but instead uses a new WP_Query. Since Relevanssi still blocks the default search, this comes up empty.
For a straightforward solution, add this to your theme functions.php:
add_filter( 'relevanssi_search_ok', function( $ok, $query ) {
if ( ! empty( $query->query_vars['s'] ) ) {
$ok = true;
}
return $ok;
}, 10, 2 );
?
This function hooks to the relevanssi_search_ok filter hook. This hook fires because Relevanssi looks at the newly-created WP_Query, but since it’s not the main query for the template, Relevanssi doesn’t take over. This function instructs Relevanssi to take over as long as there’s a s parameter in the query and a search term exists.”