When Access runs a query, it actually runs the SQL Statement. The Design View is only a graphical presentation of the SQL Statement.
When you typed Between [Enter From Date] And [Enter Thru Date] Is Null in the criteria cell in Design View, Access was confused by the two operators: Between and Is Null, but instead of popping up an error message, it made a guess and created an SQL Statement behind the scenes, without realizing it was syntactically incorrect. When you reopened the query in Design View, it just displayed the columns according to the SQL Statement that it created. That's why now you saw two columns.
And that's why I said in my first post that, for this kind of composite criteria, it would be easier to type the criteria in the Where Clause of the SQL Statement in SQL View, as we can type the correct Where Clause without making Access guess at our needs.