After creating an encryption context and assigning it to a sys_user_role, a user containing the sys_user_role is unable to see an encryption option on the attachment popup window. A checkbox labeled 'Encrypt' or 'Encrypt with context' is missing in the attachment popup window.
The current session is not updated with the new encryption context permissions.
In order for the affected user to acquire the new sys_user_role-encryption context permissions, he needs to logout an log back in. After that, he will be able to see the encryption checkbox on attachments.