Using access group to hide certain accounts/activities to other role

Summary:

We want to restrict access to certain Account records based on the value of the NationalAccounts__c field, but these accounts are still visible to all user roles despite the access group configuration.

Content (please ensure you mask any confidential information):

We have set up object-level sharing rules on the Account object using Access Groups, with the intention of restricting visibility of certain records based on the custom field NationalAccounts__c. This field is being used to determine which accounts should be hidden from specific roles.

However, during testing, users in roles that should not have access are still able to view these records. The Access Group appears to be misconfigured, or not applying the filtering as expected.