In product variant attributes while adding an attribute one can choose the control type like 'drop-down list' and can add values which get later on displayed in product details page in the control.
While adding the 'is pre-selected' option is displayed. If one pre-select any of the value it get pre-selected when displayed in product details. Whereas even one does not choose any of the values in the attribute it still pre-select the first value.
Whereas it should not pre-select any of the values and there should a validation associated with it if 'is required' option has been selected against the attribute.