This is fixed in iOS 18.4. I have implemented this workaround for version 18.0 to 18.3:

if #available(iOS 18.4, *) {
} else if #available(iOS 18, *),
          UIDevice.current.userInterfaceIdiom == .pad,
          horizontalSizeClass == .regular {

    Rectangle()
        .fill(.regularMaterial)
        .frame(height: 50)
}