Select fields are rendered differently for taxonomy fields than for regular select fields. The regular ones you can alter using the acf/load_field filter, for instance by grouping values in optgroups.
But as taxonomies are rendered using wp_list_categories() and the acf_taxonomy_field_walker, I can’t alter the selectable choices. Instead, in my custom acf/load_field filter, I had to set $field[‘type’] = ‘select’; when presenting a front-end form. Unfortunately, that started messing with the field’s real settings, which now showed up with Field Type: Select (what I did inside the filter hook seems to update the field itself).