When investigating how a user's information may have been changed, the Profile Changes on their profile does not show any modifications made within Core.
This could be due to a recent Data Refresh being performed. Because the changes are happening on the back-end instead of through the application itself, the changes will not display here. If a field has been removed and a data refresh was recently performed, check the processed file for any blank fields. If these are found, be sure to correct the database where the information was pulled from to prevent this from being changed the next time a refresh is performed.