I think this is a bug but starting here in case it isn't.
Adding Field Type = User to the Forms does not render appropriately in the UI.
I have added both system and udf User field types as well as single select and multi-user select to the Manual Test > Edit Form.
Only the multi-select udf is rendered in the UI in the same manner as I have defined in the form -- all other User field types are displayed in the 'Related User' drop down on the right.
I would like to see what I have defined in the form to be rendered the same in the UI.
Screenshot example attached.