Purchasing category mapping vs GL Security Rules

Hi,

I have a question regarding purchasing categories and how
they operate with GL security Rules (I’ve posted this in the Procurement area
but I realise there is an overlap with General ledger here).

We have a number of Purchasing categories set up  that are mapped to Natural Account Segments
(Expense codes) – Purchasing Superuser > Setup > Financials >
Accounting > Expense Account Rules.

Against a specific iProcurement responsibility we have set
up a General Ledger Security  Rule
intended to force requesters to use a single/specific Natural Account Segment
no matter which purchasing category they choose.  However, when raising a requisition this rule