Why validation of dropdown field is triggered even though the field value is null?

a!localVariables(
  local!singleDropDownValue,
  local!multipleDropDownValue,
  {
    a!dropdownField(
      label: "Single Dropdown",
      placeholder: "Select",
      choiceLabels: { "Jane", "Jack" },
      choiceValues: { 1, 2 },
      value: local!singleDropDownValue,
      saveInto: { local!singleDropDownValue },
      validations: "Error..."
    ),
    a!multipleDropdownField(
      label: "Multiple Dropdown",
      placeholder: "Select",
      labelPosition: "ABOVE",
      choiceLabels: { "Jane", "Jack" },
      choiceValues: { 1, 2 },
      value: local!multipleDropDownValue,
      saveInto: { local!multipleDropDownValue },
      validations: "Error..."
    ),
    a!buttonLayout(
      primaryButtons: a!buttonWidget(label: "submit", submit: true)
    )
  }
)

Expecting validation to trigger only when the field value is not null. It works for multiple dropdown, but why not for dropdown?