Enum default value isolation

Using Swift
,
1

I have read SE-0411 Isolated default value expressions

@MainActor
public struct TextConfig {
    public init(title: String = "") {
        self.title = title
    }
    var title: String
}

@MainActor
struct BarItemConfigStruct {
    var title: TextConfig
    static func title(config: TextConfig = TextConfig()) -> BarItemConfigStruct {
        return BarItemConfigStruct(title: config)
    }
}
 
@MainActor
public enum BarItemConfigEnum {
    case title(config: TextConfig = TextConfig()) //error: Main actor-isolated default value in a nonisolated context
}

However, I encountered an error with the above code.
I can roughly guess the reason:
the static structure of the enum causes TextConfig not to guarantee evaluation of default values within the main actor isolation.
But I think this should be supported, after all, TextConfig is isolated by the main actor.

Please point out my mistakes, I am willing to learn more details.

ENV: Xcode 16.0 (16A242d)