CiviReport Custom Fields don't work if user has been granted access to some custom fields by ACL

I have noticed 2 odd behaviours when users view custom reports and they have ACL permission to view some custom data fields but not all custom data fields

1) None of the filters for custom data fields are available to them
2) If an admin creates a report & filters by a custom data field that filter is no loaded when the report is viewed by the not-all-custom-field-viewing-user - regardless of whether they have permission to that field

#2 breaks down into 4 scenarios
1) they have permission to view the filtered field & any specific custom fields added
2) They have permission to view the filtered field but not any specific custom fields added as columns
3) They have the permission to view the specific custom fields added as columns but not the filtered field
4) They don't have permission to either

#1 is clear cut but my understanding is that in fact they should see the report as 'designed' by an admin in the other 3 scenarios too (but filtered by their ability to view specific contacts). I'm nott 100% sure about the desired behaviour in 2-4 though.