It seems to work fine to me. You should post the code you're using, describe what it does and what you would expect it to do.
In the following code, the completion propagates correctly to both targets for me:
var buffer = new BufferBlock<int>();
var target1 = new ActionBlock<int>(i => Console.WriteLine("Target 1: {0}", i));
var target2 = new ActionBlock<int>(i => Console.WriteLine("Target 2: {0}", i));
buffer.LinkTo(target1, new DataflowLinkOptions { PropagateCompletion = true });
buffer.LinkTo(target2, new DataflowLinkOptions { PropagateCompletion = true });
buffer.Post(1);
buffer.Post(2);
buffer.Complete();
Task.WaitAll(target1.Completion, target2.Completion);