The endpoint is documented as a POST request with a {"tag_id": } payload [1], but it is in fact implemented as a DELETE request:

https://github.com/mantisbt/mantisbt/blob/c9b56bcdd8c86b6acdd672dfe66ba0527e812672/api/rest/restcore/issues_rest.php#L51-L52

    $g_app->delete( '/{id}/tags/{tag_id}', 'rest_issue_tag_detach' );
    $g_app->delete( '/{id}/tags/{tag_id}/', 'rest_issue_tag_detach' );

Documentation must be updated to reflect what the code actually does.