Why is a separation read-only?
Once a separation reaches Completed or Cancelled, the form becomes read-only automatically — a banner ("Separation closed. This record is read-only.") appears, all fields are disabled, and stage-action buttons are hidden. The action bar still shows Save for ancillary edits where allowed (e.g., post-mortem notes by admins).
You can also force a read-only view by appending ?v=1 to the edit URL.