.net - Reactive Extensions - GroupBySelect many - Stack Overflow

I'm trying to get the last element of each group and then buffer the result. but applying the SelectMany with LastAsync publishes no elements to the buffer.

What am I doing wrong?

Stream.GroupBy(el => el.Id)
               .SelectMany(group => group.LastAsync())
                .Buffer(TimeSpan.FromMilliseconds(200))
            .SkipWhile(b=>!b.Any());