MailKit extensionViewController(messageContext:) nil context?

In Mail Kit, MEMessageSecurityHandler.extensionViewController(messageContext:)'s context is always nil. Any idea why?

class MessageSecurityHandler: NSObject, MEMessageSecurityHandler {

    func extensionViewController(messageContext context: Data) -> MEExtensionViewController? {
        let controller = ExampleSigningViewController.sharedInstance
        controller.msgContext = context // nil ??? why?
        return controller
    }

I thought it had to do with the MEDecodedMessage.context returned in decodedMessage(forMessageData:) but changing returned the context below still results in the above giving an empty Data.

func decodedMessage(forMessageData data: Data) -> MEDecodedMessage? {
        MEDecodedMessage(
            data: data,
            securityInformation: MEMessageSecurityInformation(
                signers: [],
                isEncrypted: true,
                signingError: nil,
                encryptionError: nil),
            context: data,   // extensionViewController(messageContext:)'s context is still empty
            banner: nil)

Env: macOS Ventura 13.4.1 (c) (22F770820d) Mail Version 16.0 (3731.600.7) Xcode 14.2