I would say that it's a bug. If you open created form as a table you'll see that 'DisplayValue=1' is stored correctly.

>1.- Create an new Form
>2.- Add a Combobox control
>3.- Change DisplayValue to 1
>4.- Save the form
>5.- Open the Form, the DisplayValue value now is 73!!
>6.- At this point there's no way to change the value, only Reseting to Default
>
>

>lcOldSafety = SET("Safety")
>SET SAFETY OFF
>ERASE Combo_test.*
>CREATE FORM Combo_Test NOWAIT
>   oForm = _VFP.ActiveForm
>   oForm.AddObject("Combo1","ComboBox")
>   *** oForm.Combo1.Style = 2
>   oForm.Combo1.DisplayValue = 1
>KEYBOARD '{CTRL+W}'
>MODIFY FORM Combo_Test NOWAIT
>  oForm = _VFP.ActiveForm
>  MESSAGEBOX("The value should be 1 y NOT:"+;
>             STR(oForm.Combo1.DisplayValue))
>SET SAFETY &lcOldSafety
>