My guess is, that Sitecore requires the Workflow Approver to Checkout the Item into Edit-Mode before manipulating (= approve/reject) it, just as if it would be a regular Content Author. Can the Workflow Approver edit the fields on the Item WITHOUT locking the item into Edit-Mode? Was the item probably not checked-in by the Content Author beforehand or when passed through the Workflow steps?
You may check if one of the following Sitecore.config settings would be useful to change/adjust to resolve the current behaviour - but keep in mind that it impacts all Content Authors working in the System.
- RequireLockBeforeEditing = false
- AutomaticUnlockOnSaved = true
Source: Sitecore Content Author’s Cookbook, Chapter 4.1.4 „Locking in the Content Editor“