are all your changes inside one single block? this part of the email template could be overwritten by a module block.
You can check the table “oxtplblock” if there are entries for block “email_html_order_owner_basketitem”.
If your changes are distributed over the whole email template but you cant see any of them, then you are probably editing the wrong file and there could be a child theme.
i would check the file on the server, just to make sure your changes were uploaded correctly.
If all the changes were uploaded correctly, my last guess would be a module, that overwrites Email class and renders a completely different template file instead of the default order_owner.tpl