I am looking at this code
var numbers = Enumerable.Range(0, 20);
var parallelResult = numbers.AsParallel().AsOrdered()
.Where(i => i % 2 == 0).AsSequential();
foreach (int i in parallelResult.Take(5))
AsSequential is just meant to stop any further parallel execution - hence the name. I'm not sure where you got the idea that it's "supposed to make the resulting array sorted". The documentation is pretty clear:
Converts a ParallelQuery into an IEnumerable to force sequential evaluation of the query.
As you say,
AsOrdered ensures ordering (for that particular sequence).