CRM 2013 Able to edit read only fields after activating a record

Has anyone else noticed this

Same on all entities we've tested so far, so ....

Create Contact with a read only field (field is read only)

deactivate the contact (field is read only still)

activate the contact (field is now editable)

close and reopen the contact record (field is read only again)

You can get around it by adding in some jScript but it shouldn't happen, any ideas?

Thanks

Pete