Adding/changing/removing a user's Content Editor role does not show up in the Role Membership history, because Content Editors belong to Groups, rather than having the role assigned/removed.

To view a user's Role Membership History, a Platform Manager can:
  1. Navigate to Core.
  2. Using People Finder, Search for the user > Select the user.
  3. Select the Access Tab.
  4. Select Role Membership
  5. Select View History  
Platform Managers can add Content Editor access to a user.