You are right, that does not work correctly. As a temporary resolution, I have made a few fixes in the current test version. If you are comfortable with editing files on your server, then please follow the guide below to resolve this. If you need help, you can add temporary FTP access and I will make the change for you.
1. Make sure to have a full server backup just in case
2. Open up the wp-content/plugins/ajax-search-pro/includes/functions/functions.php file on your server
3. Replace it’s contents with this
4. Save the file and reload the search page
The {get_values checked} should now select the first value for drop downs and radios, as well as select all values for multiselect dropdowns. The {get_values} should not select anything with radio buttons.