For Readonly Insight Custom Field, "Filter Assign Scope (IQL)" only takes affect for newly created/updated issues as the filter only gets triggered upon a IssueCreated or IssueUpdated events.
As a result, as a Jira/Insight administrator it's difficult to automatically populate the "Readonly Insight Custom Field" values for existing issues until these issues are manually updated.
- Create a few Jira issues first
- Add a Readonly Insight Custom Field and add to the relevant screens
- Add Filter Assign Scope (IQL)
Existing issues also changes the custom field value
Existing issues would not change the RO Insight custom field unless they're updated.