NavigationSuiteScaffold draws a Surface using

Surface(modifier = modifier, color = containerColor, contentColor = contentColor) { ... }

This covers the whole scaffold, including the content lambda. It's a bit unclear what the intended usage is. For example, contentColor will override the color of text in the content lambda but not the nav items (since they have their own colors set separately). It seems a bit odd to have the scaffold affect the content in that way, but I'm not sure if there was a specific intent behind this.

The default Compose templates uses this layout:

setContent {
    AppTheme {
        Surface() {
            // app stuff
        }
    }
}

So maybe the intent is to replace the default surface with the NavigationSuiteScaffold?