`NewDocumentButton(contentType:)` gives "Content serialization failed, document won't be saved."

I'm working on an iOS document-based app. It uses ReferenceFileDocument and custom creation of documents via DocumentGroupLaunchScene + NewDocumentButton. It works fine when I use the plain NewDocumentButton("Whatever") (without any more arguments), but when I want to perform additional setup via preapreDocumentURL or even just add a contentType it gives such output in the console when I hit it:

Content serialization failed, document won't be saved.

UTType.replayable is correctly wired up in the plist.

It looks like a bug in the SDK, but maybe there is a chance that I'm doing something wrong?

Here's a code:

import SwiftUI
import UniformTypeIdentifiers
import Combine

@main
struct MyApp: App {
  var body: some Scene {
    DocumentGroup {
      Document()
    } editor: { documentConfiguration in
      EmptyView()
    }
    
    DocumentGroupLaunchScene("Yoyo") {
      NewDocumentButton(contentType: .replayable) {
        return URL(string: "whatever, it doesnt even go there...")!
      }
    }
  }
}

final class Document: ReferenceFileDocument {
  static var readableContentTypes: [UTType] { [.replayable] }
  
  @Published var x = 0
  
  init() {}
  init(configuration: ReadConfiguration) throws {}
  
  func snapshot(contentType: UTType) throws -> Data {
    Data()
  }
  
  func fileWrapper(snapshot: Data, configuration: WriteConfiguration) throws -> FileWrapper {
    .init(regularFileWithContents: snapshot)
  }
}

extension UTType {
    static var replayable: UTType {
        UTType(exportedAs: "com.whatever.yo")
    }
}