The Attribute Value Protection value of 'Use only secure subpages to protect attribute values on the list' works differently in 15.5.1 and 15.8.1.
Steps to Reproduce:
Release : 15.5.1
Component : CA PPM STUDIO
The results observed on 15.8.1 are the expected results. Secure Subpages are not available on the Team views, and Display Conditions do not determine if fields are editable or not. If the user is able to view the field on the list view, and he has the appropriate staffing access rights over the team record, he will be able to edit the field as well.
The results being different in 15.5.1 appear to be the result of an old bug that has since then been fixed.