How to correctly call an iOS completionHandler block from Delphi without crashing?

I’m trying to implement this iOS delegate method in Delphi:

- (void)URLSession:(NSURLSession *)session
              task:(NSURLSessionTask *)task willBeginDelayedRequest:(NSURLRequest *)request   completionHandler:(void (NS_SWIFT_SENDABLE ^)(NSURLSessionDelayedRequestDisposition disposition,
                                                NSURLRequest * _Nullable newRequest))completionHandler API_AVAILABLE(macos(10.13), ios(11.0), watchos(4.0), tvos(11.0));

My Delphi translation looks like this:

procedure TURLSessionDelegate.URLSession(session: NSURLSession;
                                         task: NSURLSessionTask;
                                         willBeginDelayedRequest: NSURLRequest;
                                         completionHandler: Pointer);
var
  LCompletionHandlerBlock: procedure(disposition: NSURLSessionDelayedRequestDisposition;
                                     newRequest: NSURLRequest); cdecl;
begin
  @LCompletionHandlerBlock := imp_implementationWithBlock(completionHandler);
  try
    LCompletionHandlerBlock(NSURLSessionDelayedRequestCancel, nil);
  finally
    imp_removeBlock(@LCompletionHandlerBlock);
  end;
end;

the problem: it crashes on the call

LCompletionHandlerBlock(NSURLSessionDelayedRequestCancel, nil);

However, this version seems to work:

procedure TALHttpWorker.TURLSessionDelegate.URLSession(session: NSURLSession;
                                                       task: NSURLSessionTask;
                                                       willBeginDelayedRequest: NSURLRequest;
                                                       completionHandler: Pointer);
var
  LCompletionHandlerBlock: procedure(disposition: NSURLSessionDelayedRequestDisposition;
                                     zzz: Pointer;
                                     newRequest: Pointer;
                                     www: Pointer); cdecl;
begin
  @LCompletionHandlerBlock := imp_implementationWithBlock(completionHandler);
  try
    LCompletionHandlerBlock(NSURLSessionDelayedRequestUseNewRequest,
                            nil,
                            NSObjectToID(willBeginDelayedRequest),
                            nil);
  finally
    imp_removeBlock(@LCompletionHandlerBlock);
  end;
end;

If I declare the block as procedure(disposition: NSURLSessionDelayedRequestDisposition; newRequest: NSURLRequest); cdecl; → it crashes when invoked.

If I declare it as procedure(disposition: NSURLSessionDelayedRequestDisposition; zzz: Pointer; newRequest: Pointer; www: Pointer); cdecl; and pass dummy nil values for zzz and www → it works, and disposition / newRequest are correctly taken into account.

I don’t understand why I need those two extra dummy parameters to make it work. The original Objective-C block signature only has two parameters:

void (^)(NSURLSessionDelayedRequestDisposition disposition,
         NSURLRequest * _Nullable newRequest)

• Why does the “natural” Delphi translation with only two parameters crash?
• Why does adding two extra Pointer parameters (zzz / www) suddenly make it work?
• What is the correct Delphi declaration for this completionHandler block when using imp_implementationWithBlock?

Any explanation about the calling convention / block ABI here (and how to properly map it in Delphi) would be very helpful.