Returning early because this is checking for taxonomy support within Autocomplete, which is not for sure the case:
if ( ! $term || ! $this->index->supports( $term ) ) { ... }
Extra notes: Most likely need to move where we're listing to term edits, as this is post update indexing related, and not related to indexing terms as autocomplete sources.
Very likely defaulting to just post post type, as per standard WP_Query when not specifying post types.:
$args = [
'posts_per_page' => -1,
'tax_query' => [
[
'taxonomy' => $taxonomy,
'field' => 'term_id',
'terms' => $term_id,
],
],
];
Extra notes: specify post types based on enabled autocomplete/searchable_posts types.
Extra Extra Notes: See if this affects how we potentially approach #430