add_shortcode is not working in plugin where others are working

Your shortcode callback function is incorrect:

add_shortcode('load_peoplesoft_result_page', 'load_peoplesoft_results()');

You’ve included the (), but only the function name should be passed:

add_shortcode( 'load_peoplesoft_result_page', 'load_peoplesoft_results' );

Note that the second argument of add_shortcode() is a “callable”. This is a standard PHP feature, and you can read documentation for them here.