Out-of-the-box employee reference fields such as HRBP, Buddy, IT Admin, and Payroll Manager do not appear as selectable columns or filters in the Employee Data Changes report.
The reason is that these fields are provisioned through the Entity Framework, which means they are handled differently at the data model level.
The Employee Data Changes report does not integrate with the Entity Framework layer, and therefore cannot access these specific system-provisioned references.
This is by design, not a regression or defect.