Some fields can't be edited when the parent record is locked in Approval Process

Why Some Fields Cannot Be Edited

When a record is locked for approval, related list items in a master-detail relationship cannot be edited. Items in lookup relationships can still be edited.

Example

An admin creates an approval process for Opportunities. A custom "Expenses" object has a master-detail relationship with Opportunities; "Projects" and "Metrics" have lookup relationships. After an Opportunity is submitted for approval and locked, the rep can edit Projects and Metrics but not Expenses. Once approved, all related list items can be edited.