Your problem seems a little easier to solve then mine, if you see the url string when you search for a manufacturer :
https://www.XXX.com/index.php?manufacturers_id=63&main_page=index
then add the &sort to the end:
.com/index.php?manufacturers_id=63&main_page=index&sort=3d
it now properly sorts the manufacturer search listing. at least thats a place to start.
look in the Tpl ... additional_sort ...php at the
echo zen_draw_form('sorter_form', zen_href_link($_GET['main_page']), 'get');
echo zen_draw_hidden_field('main_page', $_GET['main_page']);
it looks for the main_page = XXX I think it is then saving that, maybe change the main_page to include an if statement to look for the manufacturers= the fact that it is before the main_page= is whats messing it up.
please keep in mind this is a quick rambling, and may not be exactly how it works..Im a little preoccupied with my issue:(
With my issue, if I put the &sort= or &disp_order= after the url for the product listings I get the default search order.
I might think this has something to do with the fact Im using the the 'all products' listing as a template for the rest of my site (include tpl_all_products .... instead of the original listing template.