'Can't add message' error when adding attachment to internal note - Redwood Helpdesk

Summary: We have made some Data Security Policy amendments to a custom agent role relating to attachments so that agents cannot see internal attachments on HDRs that they raise for themselves.

These changes were initially working in our test environment but since promoting into Production, our users are experiencing errors. When they try to add an attachment to an internal note they see a message 'can't add message'. Permission to insert or update the attachment is denied due to the data security rules.

Can anyone advise what might be missing causing this error? I've compared the duty roles impacted and there isn't anything obviously missing.