Some authorizations are still being created without a valid authorizer. It looks like the relevant applications also don't have sent_by.
This looks to be a result of the 'link' (T220138) and proxy authorization methods. When these are marked as approved, a post-save signal then updates it to Sent. As such, there's no request.user info on the Sent update.
I am trying to transfer a coordinator from one partner to another. In the admin interface for both affected partners, I changed the coordinator, hit the Save button, and got a "Please correct the errors below" notice because the "authorizer" field is required but not filled in for several existing authorizations for these partners.