Is NULL denotes a field that has never had a value and has no data.
certain fields have a blank value which counts as data.
try changing your search to Equals instead of Is NULL and leave the value field blank.
this should now return the results you are expecting.
Applicant search with NULL isn't returning results when it should
This article explains how to search for an applicant who has no data (Is Null doesn't work).
Written by Terence Cassidy
Updated over 2 months ago