order_by not overriding list default sort order

Note – my website is not active yet so a link will not work.
I have used Participants Database on a previous website and have enjoyed using it. On my new website however when I use order_by with pdb_list the list is shown in the order that is set in the list default sort setting. My list default sort setting is date_updated. The shortcode I am using is [pdb_list fields=”author_last, reading_level” order_by=author_last]. If I change the list default sort setting to author_last it will work, but I will need other lists ordered by another field. How can I fix this?
Thank you