The text input uses a different base widget (_ModalSettingsTile) without the divider. To match the appearance of the other widgets (That use _SettingsTile), wrap the TextInputSettingsTile in a Column and add const Divider(height: 0.0), as a child.

This change applied to the package's example:

Column(
  children: [
    TextInputSettingsTile(
      title: 'User Name',
      settingKey: 'key-user-name',
      initialValue: 'admin',
      validator: (String? username) {
        if (username != null && username.length > 3) {
          return null;
        }
        return "User Name can't be smaller than 4 letters";
      },
      borderColor: Colors.blueAccent,
      errorColor: Colors.deepOrangeAccent,
    ),
    const Divider(height: 0.0),
  ],
),