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. Select Users > Edit User Profile Data.
  3. Search/select the user.
  4. Scroll to the lower left under System Information > Role Membership.
  5. In the upper right corner, click the View History link.
Platform Managers can add Content Editor access to a user.