Custom Role read_private_posts Not Working

I've created a custom role via add_role, it's in my activation hook and is being removed on deactivation, thus I'm deactivating and reactivating my plugin to see the role permission changes. I can see that the changes are appearing (I've used the User Roles and Capabilities plugin to check) but I still can't view private page and posts when logged in.

add_role( 'clerk', 'Clerk', [ 'read' = true, 'read_private_pages' = true, 'read_private_posts' = true ] );

I've even tried adding some other permissions and see straight away that the user is able to do the things so am sure that I'm adding permissions in the correct way.

So why can my Clerk user still not see private posts and pages? What am I missing, do they need some other permission in order for it to work?