Recent selections on reference fields are stored in this table (sys_ui_recent_selection).
instanceName.service-now.com/sys_ui_recent_selection_list.do
For example: here are the recent selections for the Assigned to field on the Incident table for user System Administrator
To reset this, simply delete the records on the sys_ui_recent_selection table.
Reference: https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0747334
mySNstuff.com 