OutlinedButton BorderSide is ignored in OutlinedButtonThemeData => Borderside.color and BorderSide.width is always default

Question

I am trying to apply a custom buttonStyle inside of ThemeData for my Flutter App. Everything works fine, but the BorderSide is ignored and is always the default (BorderSide.color = Colors.black and BorderSide.width = 1.0)

How can I change the BorderSide inside Theme so it gets red with a radius of 2.0?

outlinedButtonTheme: OutlinedButtonThemeData(
  style: ButtonStyle(
    shape: MaterialStateProperty.resolveWith((states) {
      return RoundedRectangleBorder(
        // borderRadius is not ignored and gets applied to OutlinedButton => everything fine
        borderRadius: BorderRadius.zero,
        // here it gets interesting, the side-parameter is always ignored and defaults to `color: Colors.black` and `width: BorderSide.width = 1.0`
        side: BorderSide(color: Colors.red, width: 2.0),
      );
    }),
  ),
)

Answer 1

For anyone having the same issue, OutlinedButtonThemeData has a own side parameter. The BorderSide has to be applied there.